Abstract 1530: Dissecting breast cancer dormant CTC phenotypes

Tumor relapse is a clinically relevant problem in breast cancer where patients are asymptomatic because disseminated cells appear to become dormant for periods longer than 20 years and are undetectable by current clinical tools. Uncovering phenotypes of circulating tumor cells (CTCs) - the “seeds” of intractable metastasis-offers the promise to dissect CTC heterogeneity in relation to metastatic competence, to predict biomarker assessment, and to significantly improve monitoring and treatment of cancer. However, little is known about CTC biology and how CTCs differ in their capacity to circulate while maintaining a metastatic potential. We hypothesized that EpCAM-negative breast cancer CTC subsets exist, and avoid organ arrest with extreme efficiency by the concomitant presence of quiescence and stem cell properties. We collected peripheral blood of clinically diagnosed breast cancer patients with or without brain metastasis, and performed multiparametric flow cytometry to isolate EpCAM-negative CTC subsets with stem-cell properties (CD44+/CD24-), along with combinatorial expression of two neoplastic markers: urokinase plasminogen activator receptor (uPAR) and integrin beta1 (int β1). EpCAM-negative CTCs were further interrogated at a single-cell level employing DEPArray platform. Second, we were able to culture FACS-sorted CTC subsets, selected for six cell-surface expression markers (CD45-/EpCAM-negative/CD44+/CD24-/uPAR+/-/int β1+/-), as long-term in-vitro 3D CTC tumorspheres. Third, CTC subsets were interrogated for biomarker profiling and biological characteristics. We identified adhesive, proliferative and invasive properties of 3D CTC tumorspheres which were distinct per uPAR/int β1 combinatorial expression. Lastly, we performed next-generation whole-genome sequencing and mutation analyses to discover unique genomic signatures of uPAR/int β1 CTC subsets and verified as putative CTCs originally disseminated from primary breast tumor. Additional investigations are being pursued assessing the molecular and genomic characterization of uPAR/int β1 CTC subsets comprehensively. Clinical relevance of this research includes that this may enhance abilities to prospectively identify patients who may be at high-risk of developing breast cancer brain metastasis.

Citation Format: Monika Vishnoi, Sirisha Peddibhotla, Wei Yin, Zhong Xue, Antonio T. Scamardo, Goldy C. George, David S. Hong, Dario Marchetti. Dissecting breast cancer dormant CTC phenotypes. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1530.

The isolation and characterization of CTC subsets related to breast cancer dormancy

Uncovering CTCs phenotypes offer the promise to dissect their heterogeneity related to metastatic competence. CTC survival rates are highly variable and this can lead to many questions as yet unexplored properties of CTCs responsible for invasion and metastasis vs dormancy. We isolated CTC subsets from peripheral blood of patients diagnosed with or without breast cancer brain metastasis. CTC subsets were selected for EpCAM negativity but positivity for CD44(+)/CD24(-) stem cell signature; along with combinatorial expression of uPAR and int β1, two markers directly implicated in breast cancer dormancy mechanisms. CTC subsets were cultured in vitro generating 3D CTC tumorspheres which were interrogated for biomarker profiling and biological characteristics. We identified proliferative and invasive properties of 3D CTC tumorspheres distinctive upon uPAR/int β1 combinatorial expression. The molecular characterization of uPAR/int β1 CTC subsets may enhance abilities to prospectively identify patients who may be at high risk of developing BCBM.

Abstract 363: Dissecting CTC phenotypes: insights into mechanisms of breast cancer dormancy

Uncovering phenotypes of patient-derived Circulating Tumor Cells (CTCs) offers the promise to dissect CTC heterogeneity in relation to metastatic competence, and to determine biomarkers of therapeutic utility for improved treatment. However, it is still unknown whether and how CTCs differ in their capacity to circulate while maintaining metastatic potential. Rates of CTC survival are highly variable, lasting less than few hours in some patients but in the order of decades in others. This can lead to many questions for yet unexplored mechanisms of CTCs responsible for dormancy, along with their properties and biomarker functionalities.

We hypothesized that breast cancer CTC subsets possessing markers of pluripotency avoid organ arrest with extreme efficiency by the concomitant presence of quiescence and stem cell properties; and that expression of urokinase plasminogen activator receptor (uPAR) and beta-1 integrin (β1int), two biomarkers known to be directly implicated in tumor cell dormancy, are relevant in controlling the recurrence of breast cancer brain metastasis (BCBM). First, we isolated CTC subsets not expressing the epithelial cell adhesion molecule (EpCAM-negative CTCs), and characterized these subsets using DEPArrayTM, a new CTC platform able to dissect CTC heterogeneity at a single-cell level, thus interrogating the smallest functional unit of cancer. We captured EpCAM-negative/CD45-/CD44+/CD24- breast cancer CTC subsets that possessed combinatorial uPAR and β1int expression using multiparametric flow cytometry. Second, CTC subsets grew in vitro and were further characterized by DEPArrayTM. Markers expression was confirmed by confocal microscopy with subsets possessing a specific breast cancer gene profiling. Third, EpCAM-negative CTC subsets (uPAR+/β1int+ and uPAR-/β1int-) were interrogated for human embryonic stem cell markers by RT2 PCR arrays. Gene expression profiling was consistently distinct among uPAR+/β1int+ vs. uPAR-/β1int- CTC subsets and dependent upon patients’ BCBM status: expression of genes implicated in cell cycle progression (e.g., CDK42, CDK1), angiogenesis (e.g., FGF-2), and pluripotency (e.g., KLF4) was >30-fold higher than controls. Third, CTC subsets gene patterns isolated from patients with BCBM possessed RT2 profiles that were strikingly distinct from ones derived from patients with no BCBM. Of note, gene expression for RIF-1, a protein that counteracts actions of the breast cancer suppressor BRCA1, was highest (>50-fold) with distinct RIF-1 nuclear patterns in BCBM CTC subsets.

In summary, we have linked EpCAM-negative uPAR/β1int CTC subsets and their properties to clinical BCBM; and will assess the therapeutic inhibition of uPAR/β1int CTC biomarkers on BCBM development and its timing. Deciphering the relevance of uPAR/β1int as key CTC biomarkers of dormancy vs. metastatic competence will elucidate CTC mechanisms responsible for BCBM onset.

Citation Format: Sirisha Peddibhotla, Monika Vishnoi, Wei Yin, Yizhen Chen, Antonio Scamardo, David Hong, Dario Marchetti. Dissecting CTC phenotypes: insights into mechanisms of breast cancer dormancy. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 363. doi:10.1158/1538-7445.AM2015-363

Delineation of candidate genes responsible for structural brain abnormalities in patients with terminal deletions of chromosome 6q27

Patients with terminal deletions of chromosome 6q present with structural brain abnormalities including agenesis of corpus callosum, hydrocephalus, periventricular nodular heterotopia, and cerebellar malformations. The 6q27 region harbors genes that are important for the normal development of brain and delineation of a critical deletion region for structural brain abnormalities may lead to a better genotype-phenotype correlation. We conducted a detailed clinical and molecular characterization of seven unrelated patients with deletions involving chromosome 6q27. All patients had structural brain abnormalities. Using array comparative genomic hybridization, we mapped the size, extent, and genomic content of these deletions. The smallest region of overlap spans 1.7 Mb and contains DLL1, THBS2, PHF10, and C6orf70 (ERMARD) that are plausible candidates for the causation of structural brain abnormalities. Our study reiterates the importance of 6q27 region in normal development of brain and helps identify putative genes in causation of structural brain anomalies.

Abstract 3814: Exome sequencing for identification of causative genes for mosaic variegated aneuploidy

Mosaic Variegated Aneuploidy (MVA: OMIM 257300) is a rare autosomal recessive syndrome associated with growth deficiency of prenatal onset, microcephaly, intellectual disability, anomalies of the central nervous system, mild physical dysmorphic features and predisposition to cancer. Of the 37 reported cases of MVA, the cancers reported include Wilms tumor, rhabdomyosarcoma and leukemia. The cells from MVA patients demonstrate variable whole chromosome loss and gains suggesting an underlying chromosome segregation dysfunction. Previous studies have demonstrated biallelic loss-of-function mutations in mitotic spindle-assembly checkpoint gene BUB1B or a centrosomal microtubule stabilizing gene CEP57 in some MVA patients. Therefore, the goal of this project is to identify novel causative genes that contribute to the development of MVA.

We have identified one patient diagnosed with clinical features of MVA associated with myelodysplasia and acute myeloid leukemia. Cytogenetic analysis of peripheral blood karyotype demonstrated low-level mosaicism with multiple clones of various chromosomal gains such as trisomy 1, 5, 6, 8 and 18. Molecular sequence analysis confirmed normal BUB1B and CEP57 in this patient. Therefore, whole-exome sequencing is being used as an approach to identify disease-causing genes in our patient. Exome data analysis assuming an autosomal recessive model and Sanger sequencing confirmed four putative genes with biallelic mutations in AIM1 (absent in melanoma 1), EPS15 (epidermal growth factor receptor substrate 15), MICALL2 (molecule interacting with CasL-like protein 2) and ZSCAN12 (zinc finger and SCAN domain containing 12). Review of literature reveals potential roles of these genes on cytoskeleton, mitotic spindle formation and cytokinesis.

To further investigate these potential loci in our MVA patient, we designed cellular and functional assays using patient skin fibroblasts. Using interphase-FISH analysis, we have recapitulated the aneuploidy phenotype with abnormal chromosome numbers involving trisomy 8 in the patient skin fibroblasts. Moreover, immunocytochemistry with cytoskeletal and mitotic markers display aberrant anaphase with dysfunctional spindle mid-zone and cytokinesis failure. In addition, the patient fibroblasts also exhibited increased percentage of binucleated and micronucleated progeny due to abnormal chromosome segregation and cytokinesis. In summary, exome sequencing revealed a small number of potential causative genes as the etiology of MVA in our patient. Our early results demonstrate biomarkers for chromosome instability and aneuploidy such as non-disjunction, cytokinesis failure, micronuclei and binucleation in patient fibroblasts. Further work will determine whether dysregulation of these candidate genes can recapitulate this instability phenotype leading to mosaic aneuploidy in our patient.

Citation Format: Sirisha Peddibhotla, Bradford Powell, Meha P. Patel, Pulivarthi H. Rao, David A. Wheeler, Richard A. Gibbs, Sharon E. Plon. Exome sequencing for identification of causative genes for mosaic variegated aneuploidy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3814. doi:10.1158/1538-7445.AM2013-3814

P190B RhoGAP Regulates Chromosome Segregation in Cancer Cells

Rho GTPases are overexpressed and hyperactivated in many cancers, including breast cancer. Rho proteins, as well as their regulators and effectors, have been implicated in mitosis, and their altered expression promotes mitotic defects and aneuploidy. Previously, we demonstrated that p190B Rho GTPase activating protein (RhoGAP) deficiency inhibits ErbB2-induced mammary tumor formation in mice. Here we describe a novel role for p190B as a regulator of mitosis. We found that p190B localized to centrosomes during interphase and mitosis, and that it is differentially phosphorylated during mitosis. Knockdown of p190B expression in MCF-7 and Hela cells increased the incidence of aberrant microtubule-kinetochore attachments at metaphase, lagging chromosomes at anaphase, and micronucleation, all of which are indicative of aneuploidy. Cell cycle analysis of p190B deficient MCF-7 cells revealed a significant increase in apoptotic cells with a concomitant decrease in cells in G1 and S phase, suggesting that p190B deficient cells die at the G1 to S transition. Chemical inhibition of the Rac GTPase during mitosis reduced the incidence of lagging chromosomes in p190B knockdown cells to levels detected in control cells, suggesting that aberrant Rac activity in the absence of p190B promotes chromosome segregation defects. Taken together, these data suggest that p190B regulates chromosome segregation and apoptosis in cancer cells. We propose that disruption of mitosis may be one mechanism by which p190B deficiency inhibits tumorigenesis.

The DNA damage effector Chk1 kinase regulates Cdc14B nucleolar shuttling during cell cycle progression

Chk1 is a critical effector of DNA damage checkpoints necessary for the maintenance of chromosome integrity during cell cycle progression. Here we report, that Chk1 co-localized with the nucleolar marker, fibrillarin in response to radiation-induced DNA damage in human cells. Interestingly, in vitro studies using GST pull down assays identified the dual-specificity serine/threonine nucleolar phosphatase Cdc14B as a Chk1 substrate. Furthermore, Chk1, but not a kinase-dead Chk1 control, was shown to phosphorylate Cdc14B using an in vitro kinase assay. Co-immunoprecipitation experiments using exogenous Cdc14B transfected into human cells confirmed the interaction of Cdc14B and Chk1 during cell cycle. In addition, reduction of Chk1 levels via siRNA or UCN-01 treatment demonstrated that Chk1 activation following DNA damage was required for Cdc14B export from the nucleolus. These studies have revealed a novel interplay between Chk1 kinase and Cdc14B phosphatase involving radiation-induced nucleolar shuttling to facilitate error-free cell cycle progression and prevent genomic instability.

Thermal enhancement with optically activated gold nanoshells sensitizes breast cancer stem cells to radiation therapy

Breast cancer metastasis and disease recurrence are hypothesized to result from residual cancer stem cells, also referred to as tumor-initiating cells, which evade initial treatment. Using both syngeneic mouse and human xenograft models of triple-negative breast cancer, we have demonstrated that a subpopulation enriched in cancer stem cells was more resistant to treatment with 6 gray of ionizing radiation than the bulk of the tumor cells, and accordingly their relative proportion increased 48 to 72 hours after ionizing radiation treatment. In contrast, we achieved a larger reduction in tumor size without a concomitant increase in the percentage of cancer stem cells by treating with local hyperthermia for 20 minutes at 42°C after ionizing radiation using intravenously administered, optically activated gold nanoshells. Forty-eight hours after treatment, cells derived from the tumors treated with ionizing radiation plus hyperthermia exhibited both a marked decrease in tumorigenicity and a more differentiated phenotype than mock- and ionizing radiation-treated tumors. Thus, we have confirmed that these cancer stem cells are responsible for accelerated repopulation in vivo and demonstrated that hyperthermia sensitizes this cell population to radiation treatment. These findings suggest that local hyperthermia delivered by gold nanoshells plus radiation can eliminate radioresistant breast cancer stem cells.

miR-145-dependent targeting of junctional adhesion molecule A and modulation of fascin expression are associated with reduced breast cancer cell motility and invasiveness

Micro RNAs are small non-coding RNAs, which regulate fundamental cellular and developmental processes at the transcriptional and translational level. In breast cancer, miR-145 expression is downregulated compared with healthy control tissue. As several predicted targets of miR-145 potentially regulate cell motility, we aimed at investigating a potential role for miR-145 in breast cancer cell motility and invasiveness. Assisted by Affymetrix array technology, we demonstrate that overexpression of miR-145 in MDA-MB-231, MCF-7, MDA-MB-468 and SK-BR-3 breast cancer cells and in Ishikawa endometrial carcinoma cells leads to a downregulation of the cell-cell adhesion protein JAM-A and of the actin bundling protein fascin. Moreover, podocalyxin and Serpin E1 mRNA levels were downregulated, and gamma-actin, transgelin and MYL9 were upregulated upon miR-145 overexpression. These miR-145-dependent expression changes drastically decreased cancer cell motility, as revealed by time-lapse video microscopy, scratch wound closure assays and matrigel invasion assays. Immunofluorescence microscopy demonstrated restructuring of the actin cytoskeleton and a change in cell morphology by miR-145 overexpression, resulting in a more cortical actin distribution, and reduced actin stress fiber and filopodia formation. Nuclear rotation was observed in 10% of the pre-miR-145 transfected MDA-MB-231 cells, accompanied by a reduction of perinuclear actin. Luciferase activation assays confirmed direct miR-145-dependent regulation of the 3'UTR of JAM-A, whereas siRNA-mediated knockdown of JAM-A expression resulted in decreased motility and invasiveness of MDA-MB-231 and MCF-7 breast cancer cells. Our data identify JAM-A and fascin as novel targets of miR-145, firmly establishing a role for miR-145 in modulating breast cancer cell motility. Our data provide a rationale for future miR-145-targeted approaches of antimetastatic cancer therapy.

Abstract 16: Thermal enhancement with optically activated gold nanoshells sensitizes breast cancer stem cells to radiation therapy

Background Metastatic disease and recurrence are hypothesized to result, at least in part, from the residual cancer stem cells (CSCs), also referred to as tumor-initiating cells (TICs), which evade initial treatment. In several genetically engineered mouse (GEM) models that resemble human breast cancer, the tumor subpopulation CD29+/CD24+/lin− has been shown to be more tumorigenic as compared to other subpopulations by both in vitro and in vivo assays. We and others have demonstrated that the tumorigenic CD29+/CD24+/lin− subpopulation repairs radiation-induced DNA strand breaks more efficiently than the other subpopulations. Therefore, we hypothesized that hyperthermia, known to inhibit DNA repair, may radiosensitize CSCs to radiation therapy (RT) by delaying and inhibiting the repair of DNA strand breaks. Methods To test this hypothesis, we assayed for tumorigenic CD29+/CD24+/lin− cells in a p53null mouse mammary tumor model with and without radiation, and in combination with hyperthermia (42°C) administered using optically-activated gold nanoshells. We then determined the percentage of functional CSCs by FACS analysis and limiting dilution transplantation assays. Results These studies confirmed that CD29+/CD24+/lin− cells were more radiation resistant and that their relative proportion was increased after RT, as compared to the other cell populations. In contrast, post-irradiation treatment with 20 minutes of local hyperthermia, using intravenously administered optically activated gold nanoshells, a larger reduction in tumor size was observed, without a concomitant increase in the percentage of CD29+/CD24+/lin− tumorigenic cells. Using functional limiting dilution transplantation assays to assay for CSCs, we demonstrated that 48 hours after treatment cells derived from tumors treated with RT and hyperthermia exhibited a decreased frequency of TICs compared to untreated controls, while in contrast tumors treated with RT alone exhibited an increase in TIC frequency. Thus, there was a 20-fold difference in TIC frequency between the RT only, and RT and hyperthermia treated tumors. Importantly some of the tumors that grew out after radiation and hyperthermia treatment were less aggressive histologically as compared to those treated with radiation alone, which actually displayed a more aggressive histology than mock treated tumors. These results obtained with the GEM model were validated using two different primary triple-negative breast cancer xenografts. Conclusion These results suggest that localized hyperthermia administered using optically-active gold nanoshells serves as a simple strategy to selectively sensitize radioresistant CSCs to enhance their response to radiation therapy. Supported by grants NCI R01 CA112305 and CA16303.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 16.

Chk1 haploinsufficiency results in anemia and defective erythropoiesis

BACKGROUND

Erythropoiesis is a highly regulated and well-characterized developmental process responsible for providing the oxygen transport system of the body. However, few of the mechanisms involved in this process have been elucidated. Checkpoint Kinase 1 (Chk1) is best known for its role in the cell cycle and DNA damage pathways, and it has been shown to play a part in several pathways which when disrupted can lead to anemia.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we show that haploinsufficiency of Chk1 results in 30% of mice developing anemia within the first year of life. The anemic Chk1+/- mice exhibit distorted spleen and bone marrow architecture, and abnormal erythroid progenitors. Furthermore, Chk1+/- erythroid progenitors exhibit an increase in spontaneous DNA damage foci and improper contractile actin ring formation resulting in aberrant enucleation during erythropoiesis. A decrease in Chk1 RNA has also been observed in patients with refractory anemia with excess blasts, further supporting a role for Chk1 in clinical anemia.

CONCLUSIONS/SIGNIFICANCE

Clinical trials of Chk1 inhibitors are currently underway to treat cancer, and thus it will be important to track the effects of these drugs on red blood cell development over an extended period. Our results support a role for Chk1 in maintaining the balance between erythroid progenitors and enucleated erythroid cells during differentiation. We show disruptions in Chk1 levels can lead to anemia.

Abstract B21: Defining the DNA damage response-mediated barrier to ErbB2-induced transformation of somatic mammary cells in vivo

The DNA damage response (DDR) represents a signaling cascade, which is comprised of DNA damage sensors (such as γH2AX and 53BP1), ATM, Chk2, p53, among others, that becomes activated when confronted with DNA double strand breaks and other forms of DNA damage. The activation of the DDR, in addition to the ARF pathway, has recently been found to elicit apoptosis and senescence in response to oncogene activation. However, the significance of a DDR in blocking cancer progression has not been tested in breast carcinogenesis. Preneoplastic lesions in four germline transgenic models of breast cancer that express polyoma middle T antigen (PyMT), ErbB2, c-Myc, or H-Ras do not exhibit a DDR. We have previously reported a novel mouse model of mammary cancer, where oncogenes can be introduced into somatic mammary epithelial cells via retroviral infection of RCAS vectors into MMTV-tva mice (termed RCAS-TVA). Hyperplastic lesions in the mammary gland induced by somatic introduction of PyMT using the RCAS-TVA system display a robust DDR accompanied by increased p53 and apoptosis. This observation represents the first study, to our knowledge, to fully recapitulate DDR signaling in response to acute oncogenic stress in the in vivo mammary gland.

20–30% of all breast cancers display amplification of ErbB2, a cellular oncogene which, when activated in somatic murine mammary cells, forms sporadic tumors with a long latency. Like RCAS-PyMT, we found that p53 stabilization and apoptosis, as well as senescence, are induced by somatic activation of RCAS-ErbB2. In advanced RCAS-ErbB2-induced tumors, DDR signaling and ARF induction persist, yet p53 stabilization and apoptosis are lost. Surprisingly, attributes of senescence remain in some of the ErbB2 tumor cells. To demonstrate that the DDR is critical for oncogene-induced apoptosis and senescence, we generated RCAS-ErbB2 lesions in ATM-deficient mice. Strikingly, p53 stabilization, apoptosis, and senescence were no longer activated following ErbB2 activation in the absence of ATM signaling. These data suggest that a DDR plays a critical role in inhibiting breast carcinogenesis by inducing senescence and apoptosis, the latter, but not the former, of which is lost in progression to tumors. Furthermore, these findings indicate that therapies designed to bolster the activity of DDR signaling kinases and effectors may have a broad role in breast cancer prevention.

Citation Information: Cancer Res 2009;69(23 Suppl):B21.

Chking and executing cell division to prevent genomic instability

Eukaryotic cell division is an orderly and timely process involving the error-free segregation of chromosomes and cytoplasmic components to give rise to two separate daughter cells. Defects in genome maintenance mechanisms such as cell cycle checkpoints and DNA repair can impact the segregation of the genome during mitosis leading to multiple chromosomal imbalances. In mammals, the DNA damage checkpoint effector Checkpoint Kinase 1 (Chk1) is essential for responses to DNA replication errors, external DNA damage, and chromatin breaks. We reported recently that Chk1 also was essential for chromosome segregation and completion of cytokinesis to prevent genomic instability. Our studies demonstrated that Chk1 deficiency in mitotic cells causes chromosome mis-alignment, lagging chromosomes, chromosome mis-segregation, cytokinetic regression and binucleation. In addition, abrogation of Chk1 resulted in aberrant localization of mitotic Aurora B kinase at the metaphase plate, anaphase spindle midzone, and cytokinetic midbody as studied both in various cell lines and in a mouse model. Therefore, inappropriate regulation of Chk1 levels during cell cycle progression will result in failed cell division and enhanced genomic instability.

The DNA-damage effector checkpoint kinase 1 is essential for chromosome segregation and cytokinesis

Defective genome maintenance mechanisms, involving DNA repair and cell-cycle checkpoint pathways, initiate genetic instability in many sporadic and hereditary cancers. The DNA damage effector Checkpoint kinase 1 (Chk1) is a critical component of DNA replication, intra-S phase, and G(2)/M phase checkpoints and a recently reported mitotic spindle-assembly checkpoint. Here, we report for the first time that haploinsufficiency of Chk1 in mice resulted in multiple mitotic defects and enhanced binucleation. We observed that Aurora B, a critical cytokinetic regulator and a recently identified Chk1 substrate, was mislocalized in mitotic Chk1(+/-) mammary epithelia. Chk1 also exhibited distinct mitotic localization patterns and was active during unperturbed mitosis and cytokinesis in mammalian cells. Active Chk1 expression was not dependent on treatment with spindle poisons such as colcemid during mitosis and cytokinesis. Furthermore, two different complementary approaches demonstrated that abrogation of Chk1 in mitotic mammalian cells resulted in cytokinetic regression and binucleation, increased chromosome lagging and/or nondisjunction, and abnormal localization of Aurora B at late mitotic structures. Thus, Chk1 is a multifunctional kinase that serves as a nexus between the DNA damage response and the mitotic exit pathways during cell-cycle progression to prevent genomic instability and cancer.

Wnt/beta-catenin mediates radiation resistance of Sca1+ progenitors in an immortalized mammary gland cell line

The COMMA-Dbeta-geo cell line has been shown to contain a permanent subpopulation of progenitor cells that are enriched in outgrowth potential. Using the COMMA-Dbeta-geo cell line as a model, we sought to study the radioresistance of mammary progenitor cells. Using the putative progenitor cell marker stem cell antigen 1 (Sca1), we were able to isolate a discrete subpopulation of Sca1(+) multipotent cells from the immortalized COMMA-Dbeta-geo murine mammary cell line. At a clinically relevant dose, the Sca1(+) cells were resistant to radiation (2 Gy). Sca1(+) cells contained fewer gamma-H2AX(+) DNA damage foci following irradiation, displayed higher levels of endogenous beta-catenin, and selectively upregulated survivin after radiation. Expression of active beta-catenin enhanced self-renewal preferentially in the Sca1(+) cells, whereas suppressing beta-catenin with a dominant negative, beta-engrailed, decreased self-renewal of the Sca1(+) cells. Understanding the radioresistance of progenitor cells may be an important factor in improving the treatment of cancer. The COMMA-Dbeta-geo cell line may provide a useful model to study the signaling pathways that control mammary progenitor cell regulation.