Search

Medullary thyroid cancer (MTC) is an aggressive tumor with frequent lymph node and distant metastasis at diagnosis. Activating mutations of the RET receptor tyrosine kinase occur frequently and prevent apoptosis through inhibition of ATF4, a key transcriptional regulator of endoplasmic reticulum stress. Prior studies have demonstrated shRNA depletion of ATF4 increases RET protein levels and cause resistance to tyrosine kinase inhibitor-mediated cell death. Conversely, forced expression of ATF4 in MTC cells promotes RET ubiquitination and degradation. Here, we report the anti-cancer efficacy of ONC201 in MTC models. ONC201 is a selective DRD2 antagonist that is in clinical trials for a range of advanced cancers. Downstream signaling studies have found that ONC201 induces ATF expression and activates the integrated stress response (ISR) in a variety of cancers. ONC201 decreased cell survival of MTC cell lines (TT and MZCRC1) harboring activating RET mutations and increased the percentage of sub-G1 DNA content. Moreover, ONC201 decreased, in a dose-dependent manner, the abundance of RET protein and induced the mRNA and protein levels of ATF4 and its target genes. In our attempt to uncover RET targets, we performed reverse phase protein array analysis using TT- shRNA-RET knockdown cells. Among the top 5 proteins regulated by RET in this screen, we found insulin-like growth factor-binding protein IGFBP2, to be downregulated in shRNA-RET cells. ELISA based cytokine array of MTC-conditioned media showed high levels of secreted IGFBP2. IGFBP2 have been reported to be an oncogene in many human cancers and we show that elevated IGFBP2 protein levels are associated with poor prognosis of primary MTC. The treatment of MTC cells with ONC201 or overexpression of ATF4 inhibited the secretion of IGFBP2 into the media and decreased IGFBP2 protein levels, suggesting that ONC201 targets IGF signaling through activation of ISR. To investigate the efficacy of ONC201 in vivo, established MTC xenografts (TT or MZCRC1) were treated with a weekly dose of 120 mg/kg orally for 8 weeks and tumor burden was assessed every week. We observed that ONC201 decreased tumor growth of TT and MZCRC1 xenografts by 80-90%. There was no evidence of toxicity, as indicated by weight loss in mice. Immunohistochemical and western blot analysis of tumors at the termination of the study demonstrated decreased IGFBP2 protein levels in mice treated with ONC201. These studies identify IGFBP2 as a novel target of the RET proto-oncogene and demonstrate that ONC201, through its effect on ATF4, reverses RET-mediated signaling. These results support the further investigation of ONC201 as a therapeutic agent for the treatment of MTC alone or in combination with tyrosine kinase inhibitors. Citation Format: Rozita Bagheri-Yarmand, Ling Li, Rohinton Tarapore, Joshua E. Allen, Steven I. Sherman, Robert F. Gagel. ONC201 inhibits RET and IGFBP2 signaling through ATF4 mediated- Integrated stress response in medullary thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1286.

Medullary thyroid carcinoma (MTC) is a neuroendocrine malignancy, caused by activating mutations of the RET proto-oncogene, with frequent lymph node and distant metastasis at diagnosis. Tyrosine kinase inhibitors (TKIs) that target the RET have proven effective for treatment of MTC, but resistance to TKIs evolves in half of the treated patients. We have previously shown that the expression levels of Activating Transcription Factor 4 (ATF4) which is a stress-induced transcription factor, significantly decreased or lost in half of the MTC tumors. We also demonstrated that forced expression of ATF4 or TKIs-induced level of ATF4 decreases survival of MTC cells by promoting degradation of RET, blocking the activation of RET downstream signaling pathways and subsequently induces apoptosis. More importantly, ATF4 knockdown by shRNA decreased the sensitivity to tyrosine kinase inhibitor-induced apoptosis. Further thyroid gland of Atf4-knockout mice showed C-cell hyperplasia, a precancerous lesion for MTC suggesting a potential tumor suppressor role of ATF4 in MTC. Stress-induced by the accumulation of unfolded proteins in the endoplasmic reticulum (ER) is a feature of specialized secretory cells, including MTC cells that secrete calcitonin. ATF4 promotes the induction of apoptosis under persistent stress conditions, although the mechanism is not clearly understood. We hypothesize that the combination of ATF4 inducer and TKIs causes an excessive cellular oxidative stress resulting in an activation of apoptosis, all of which may prevent resistance to TKIs. Here, we show that the ER-associated protein degradation (ERAD) inhibitor, eeyarestatin sensitizes MTC cells to the TKIs including sunitinib and vandetanib. The combination of eeyarestatin and TKIs causes a synergistic induction of ATF4 expression and its target genes, an accumulation of reactive oxygen species and subsequent cell death. Chromatin immunoprecipitation followed by sequencing assay (ChIP-seq) in MTC cells treated with eeyarestatin and immunoprecipitated with ATF4 and acetylated lysine 9 of histone 3, identified transcription Kruppel-like factor 9 (KLF9) gene as a transcriptional target of ATF4 activation. KLF9 plays a functional role in oxidative stress-induced cell death. Treatment with eeyarestatin and vandetanib alone or in combination increases the occupancy of ATF4 at the promoter of KLF9 gene and stimulates the expression of KLF9. Depletion of ATF4 by shRNA leads to downregulation of KLF9 expression and prevents oxidative stress-induced cell death. These findings suggest that induction of the ATF4/KLF9 axis causes oxidative stress leading to excessive ER stress, and subsequent cell death. Thus, combining TKIs and ERAD inhibitors that promote ATF4 levels could be effective therapeutic strategies for treating MTC and preventing resistance to TKIs. Citation Format: Rozita Bagheri-Yarmand, Krishna M. Sinha, Ling Li, Yue Lu, Robert F. Gagel. Tyrosine kinase and ERAD inhibitors promote oxidative stress-induced apoptosis through activation of ATF4 / KLF9 axis in medullary thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2426.

The cyclin E–cyclin-dependent kinase 2 (CDK2) complex accelerates entry into the S phase of the cell cycle and promotes polyploidy, which may contribute to genomic instability in cancer cells. The effect of low molecular weight isoforms of cyclin E (LMW-E) overexpression on mitotic progression and its link to genomic instability were the focus of this study. Here, we show that full-length cyclin E (EL) and LMW-E overexpression impairs the G2-M transition differently by targeting dual-specificity phosphatase Cdc25C activity. We identify Cdc25C as an interaction partner and substrate for cyclin E/CDK2 kinase. Specifically, the cyclin E/CDK2 complex phosphorylates Cdc25C on Ser214, leading to its premature activation, which coincides with higher cyclin B/CDK1 and Polo-like kinase 1 (PLK1) activities in an S-phase–enriched population that result in faster mitotic entry. Whereas EL overexpression leads to hyperactivation of Cdc25C, cyclin B/CDK1, and PLK1 in a G2-M–enriched population, LMW-E overexpression causes premature inactivation of Cdc25C and PLK1, leading to faster mitotic exit. In addition, LMW-E–overexpressing cells showed a reduction in the mitotic index in the presence of a spindle poison and faster degradation of cyclin B, suggesting an increased rate of mitotic slippage and adaptation to the spindle checkpoint. Lastly, downregulation of Cdc25C inhibits LMW-E–mediated chromosome missegregation, anaphase bridges, and centrosome amplification. These results suggest that the high levels of LMW-E isoforms found in breast cancer may contribute to cellular transformation and genomic instability by impairing mitotic progression involving Cdc25C. Cancer Res; 70(12); 5085–95. ©2010 AACR.

Overexpression of the low molecular weight isoforms (LMW-E) of cyclin E induces chromosome instability; however, the degree to which these tumor-specific forms cause genomic instability differs from that of full-length cyclin E (EL), and the underlying mechanism(s) has yet to be elucidated. Here, we show that EL and LMW-E overexpression impairs the G2-M transition differently and leads to different degrees of chromosome instability in a breast cancer model system. First, the most significant difference is that EL overexpression prolongs cell cycle arrest in prometaphase, whereas LMW-E overexpression reduces the length of mitosis and accelerates mitotic exit. Second, LMW-E–overexpressing cells are binucleated or multinucleated with amplified centrosomes, whereas EL-overexpressing cells have the normal complement of centrosomes. Third, LMW-E overexpression causes mitotic defects, chromosome missegregation during metaphase, and anaphase bridges during anaphase, most of which are not detected on EL induction. LMW-E induces additional mitotic defects in cooperation with p53 loss in both normal and tumor cells. Fourth, LMW-E–overexpressing cells fail to arrest in the presence of nocodazole. Collectively, the mitotic defects mediated by LMW-E induction led to failed cytokinesis and polyploidy, suggesting that LMW-E expression primes cells to accrue chromosomal instability by shortening the length of mitosis. Lastly, LMW-E expression in human breast cancer tissues correlates with centrosome amplification and higher nuclear grade. These results suggest that LMW-E overexpression leads to higher centrosome numbers in breast cancer, which is a prerequisite for genomic instability. Cancer Res; 70(12); 5074–84. ©2010 AACR.

Metastasis-associated protein 1 (MTA1), a component of the nuclear remodeling complex and the founding homologue of the MTA family, has been implicated in metastasis, but definitive causative evidence in an animal model system is currently lacking. Here, we show that MTA1 overexpression in transgenic mice is accompanied by a high incidence of spontaneous B cell lymphomas including diffuse large B cell lymphomas (DLBCL). Lymphocytes and lymphoma cells from MTA1-TG mice are hyperproliferative. Lymphomas were transplantable and of clonal origin and were characterized by down-regulation of p27Kip1 as well as up-regulation of Bcl2 and cyclin D1. The significance of these murine studies was established by evidence showing a widespread up-regulation of MTA1 in DLBCL from humans. These findings reveal a previously unrecognized role for the MTA1 pathway in the development of spontaneous B cell lymphomas, and offer a potential therapeutic target in B cell lymphomas. These observations suggest that MTA1-TG mice represent a new model of spontaneous DLBCL associated with high tumor incidence and could be used for therapeutic intervention studies. [Cancer Res 2007;67(15):7062–7]

The activating transcription factor 4 (ATF4) plays a central role in activation of the integrated stress response pathway. Targeted deletion of Atf4 in mouse thyroid gland causes C-cell hyperplasia, a precancerous lesion for medullary thyroid cancer (MTC). Activating mutations of the tyrosine kinase RET are associated with oncogenic function in MTC progression. We have reported that RET is a serine-threonine dual specificity kinase, phosphorylates ATF4, and inhibits expression of the ATF4 target proapoptotic genes. Forced ATF4 expression in MTC cells decreased their survival through ubiquitin-mediated degradation of RET protein levels. We hypothesize that inactivation of ATF4 by RET is a central impediment to induction of apoptosis in response to TKI therapy. Further upregulation of ATF4 would render tumor cells exquisitely sensitive to stress induced apoptosis. Here, we show that the treatment of MTC cells with tyrosine kinase inhibitors upregulates ATF4 expression and induces Endoplasmic reticulum (ER) stress. Eeyarestatin is an ER-associated protein degradation (ERAD) inhibitor that elicit an integrated stress response program at the ER, which results in activation of ATF4 to stimulate expression of the apoptotic genes, PMAIP1 and BBC3, resulting into apoptotic cell death. We found that treatment of MTC cell lines, TT and MZCRC1 cells, with eeyarestatin not only induces ATF4 but also downregulates RET protein levels. The combinations of TKIs (vandetanib, sunitinib, cabozantinib) with eeyarestatin promote apoptotic cell death in a synergistic manner shown by cleaved PARP/ cleaved casapase3 staining using flow cytometry. Western blot analysis showed that combination therapy increased ATF4 levels and its targets, NOXA and PUMA levels as compared to each drug treatment alone. We applied genome-wide Chromatin Immunoprecipation Sequencing (ChIP-seq) to identify genes directly regulated by ATF4 in eeyarestatin-treated MZCRC1 cells. ChIP-seq analysis identified many previously known and unknown ATF4 target genes. We have identified the stress response, proapoptotic genes, inhibitors of MAPK, mTOR and βcatenin-wnt pathways with tumor suppressor function, including MKNK2, TCF3, APC2, KLF9, LZTFL1, SIAH1. Gene expression analysis was performed to further validate the activation of those genes in ATF4-overexpressing and eeyarestatin-treated cells. We found that combination of TKI and eeyarestatin synergistically upregulated expression of ATF4, and its targets APC2, LZTFL1, BBC3, MKNK2, and SIAH1. These results suggest that induction of ATF4 in combination with TKIs could be a promising therapeutic strategy to treat patients with medullary thyroid cancer. Citation Format: Rozita Bagheri-Yarmand, Krishna Sinha, Yue Lu, Robert Gagel. Tyrosine kinase and ERAD inhibitors synergize to promote apoptosis through ATF4 induction in medullary thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-112. doi:10.1158/1538-7445.AM2017-LB-112

Medullary thyroid carcinoma (MTC) is derived from the calcitonin-producing C-cells of the thyroid gland. Oncogenic mutations of the RET proto-oncogene are found in all hereditary MTC and ∼45% of the sporadic cases. However, it is clear that additional genetic lesions contribute to the development of both RET-dependent and RET independent MTC. We analyzed 30 primary medullary thyroid cancer (MTC) tumors by comparative genomic hybridization (CGH) arrays (Agilent 244K) and a subset of those tumors by ultra-high-resolution SNP arrays (Affymetrix SNP6.0). A consistent finding with both arrays was a loss localized to 22q13.1 in 40% of tumors. This region contains the ATF4 gene and loss of this gene was validated by real time PCR. ATF4 is a central mediator of integrated stress response pathway and activates expression of a cohort of downstream target genes known to be involved in cell survival, apoptosis, autophagy and senescence. We hypothesize that the loss of ATF4 expression contributes to MTC progression through mechanisms involving aberrant proliferation and escape from apoptosis. We present several findings to support this. First, ATF4 protein levels were found downregulated in 50% MTC tumor tissues (n = 43) and low ATF4 expression was associated with poor overall survival in MTC patients (HR = 5.015, 95% CI: 1.38-10.64), P = 0.013). A negative correlation was observed between RET and ATF4 protein expression in MTC tumors (r = -0.89, R square = 0.799, p Citation Format: Rozita Bagheri-Yarmand, Gilbert J. Cote, Elizabeth G. Grubbs, Michelle D. Williams, Steven I. Sherman, Robert F. Gagel. ATF4 is a putative tumor suppressor gene in medullary thyroid cancer. [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 3660.

Androgen ablation by pharmacological use or castration is a standard therapy for treating prostate cancer (PCa), however, at the later stage PCa becomes castration-resistant and is metastasized primarily to bones. Epigenetic changes that cause dysregulated gene expression during progression and metastasis of androgen-independent prostate cancer are not clearly understood. We previously reported that the jumonji C domain-containing NO66 is a histone demethylase specific for H3K4me3 and H3K36me3, which negatively regulates osteoblast differentiation and bone formation. By analyzing a multi-cancer tissue microarray, the serendipitous observation was made that NO66 is overexpressed in a variety of cancers. Our objective was to examine NO66's role in the pathogenesis of prostate cancer and bone metastasis. Our findings indicate that NO66 knockdown resulted in significant decreases in the proliferation and invasiveness of prostate cancer cells (PC3 and DU145) while overexpressing NO66 promoted their proliferation and invasion. Furthermore, NO66 depletion resulted in a two- to three fold decrease in colony formation and anchorage-independent growth of PCa cells suggesting an oncogenic function of NO66. RNAseq analysis comparing between control and NO66-depleted DU-145 cells revealed a repertoire of differentially expressed genes. NO66 up-regulates genes with proliferative activity such as chemokine CXCL5, growth factor IGFBP5, IL6, WNT and MAPK4 pathway, which are known to drive PCa proliferation. Furthermore, ChIP sequencing for NO66 and H3K9AC in NO66-overexpressing PC3 cells revealed that interaction sites of NO66 overlap with peaks of H3K9AC in majority of the genes. These data clearly indicate that NO66 activates a cohort of genes which are involved in tumor progression and metastasis. Strikingly, implanting NO66-overexpressing prostate cancer cells into the femur of SCID male mice caused massive femoral bone loss after three weeks, which suggests that NO66 causes osteolytic bone lesions similar to the clinico-pathological features of advanced PCa. Finally, analysis of tumor xenografts derived from patients by immunohistochemistry showed high expression of NO66 in high grade advanced PCa compared to low grade and hyperplasia suggesting that NO66 may be used as a biomarker to distinguish aggressive prostate cancer from indolent stage. Our data suggest that NO66 plays an important role in cell proliferation, cell invasion and bone metastasis of prostate cancer. Citation Format: Rozita Bagheri-Yarmand, Nora Navone, Xinhai Wan, Christopher Logothetis, Johnny Huard, Krishna Sinha. Oncogenic and osteolytic function of histone demethylase NO66 in castration resistant prostate cancer. [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 693.

Previously, we have shown that metastasis-associated protein 1 (MTA1) overexpression in transgenic mice was accompanied by high incidence of spontaneous B-cell lymphomas including diffuse large B-cell lymphomas (DLBCL). To understand the molecular basis of lymphoma in MTA1-transgenic (MTA1-TG) mice, we wished to identify a putative MTA1 target with a causal role in B-cell lymphogenesis. Using chromatin immunoprecipitation assays, we identified paired box gene 5 (Pax5), a molecule previously implicated in B-cell lymphogenesis, as a potential downstream effector of MTA1. Lymphomas from MTA1-TG mice also showed up-regulation of Pax5. We also found that MTA1 acetylated on Lys626 interacted with p300 histone acetyltransferase, and that acetylated MTA1 was recruited to the Pax5 promoter to stimulate Pax5 transcription. Global gene profiling identified down-regulation of a set of genes, including those downstream of Pax5 and directly implicated in the B-cell lymphogenesis. Significance of these murine studies was established by evidence showing a widespread up-regulation of both MTA1 and Pax5 in DLBCL from humans. These observations provide in vivo genetic evidence for a role of MTA1 in lymphomagenesis. [Cancer Res 2007;67(15):7132–8]

RET proto-oncogene, a receptor tyrosine kinase (RTK), activates a complex network of signal transduction pathways that when unregulated contributes to cell transformation. Germ line missense mutations, which give rise to constitutively active oncogenic RET, were found to cause multiple endocrine neoplasia type 2 (MEN2), a dominant inherited cancer syndrome that affects multiple neuroendocrine organs. Recent findings also suggest that impairment of RTK down-regulation or deactivation may be involved in tumorigenesis. The molecular mechanism that functions to attenuate RET signaling remain elusive. ATF4 is a central mediator of integrated stress response pathway. Activation of ATF4 results in tumor cell death under stress conditions thereby suggesting that ATF4 agonists constitute a potential therapeutic strategy for inhibiting tumor growth. Here, we report that ATF4 overexpression in medullary thyroid cancer cells harboring RET C634W mutation decreases RET protein expression but not the mRNA levels. In addition, ATF4 accelerates RET turnover and promotes its ubiquitination. Moreover, a RET kinase dead mutant is more stable than wild type or M918T RET suggesting that kinase activity is required for ATF4-mediated degradation. Finally, ATF4 depletion upregulates RET protein levels and RET autophosphorylation. Our findings unravel ATF4 as a feedback negative attenuator of RET signaling that could be used as a potential therapeutic target for medullary thyroid cancer or other malignancy with RET abnormalities. Citation Format: Rozita Bagheri-Yarmand, Gilbert J. Cote, Robert F. Gagel. Activating transcription factor 4 regulates RET autophosphorylation and ubiquitin-dependent degradation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5262. doi:10.1158/1538-7445.AM2014-5262

One of the hallmarks of human cancers is the intrinsic or acquired resistance to apoptosis. Apoptosis resistance contributes to carcinogenesis, tumor progression, and treatment resistance. Patients with medullary thyroid carcinoma (MTC) have been shown to be resistant to standard chemotherapy and conventional radiotherapy. Dominant activating mutations in the RET protooncogene are the cause of hereditary MTC and are found in more than 40% of sporadic cases. Despite an ever-growing wealth of information about RET's role in cell survival/proliferation, a detailed understanding of how this tyrosine kinase receptor acts to prevent cell death in tumorigenesis remains elusive. We hypothesized that activated RET prevents apoptosis by repressing the transcription of the proapoptotic gene Noxa in the nucleus in response to genotoxic or endoplasmic reticulum (ER) stress. Here we report that activated RET translocates to the nucleus in cultured MTC cells as well as MTC primary tumor tissues. RET is recruited to the Noxa promoter and is able to repress its transcription. The formation of the RET-Noxa promoter complex depends on RET expression and requires the tyrosine kinase activity of RET. Additionally, stable depletion of RET in MTC cells, using lentiviral shRNA, sensitizes MTC cells to genotoxic or endoplasmic reticulum stress. We specifically show that RET is recruited to the Activating Transcription Factor 4 (ATF4) binding site on the Noxa promoter. RET physically interacts with ATF4 and phosphorylates ATF4 leading to the blockage of its transcriptional activity. Moreover, RET knockdown is associated with increased ATF4-dependent expression of Noxa. Inhibition of RET kinase activity by sunitinib results in ATF4 activation, induction of Noxa expression and apoptosis. Conversely, silencing of ATF4 in MTC cells decreased sunitinib-mediated Noxa induction and apoptosis. ATF4 protein levels markedly decreased and/or localized in the cytoplasm of 60% of primary MTC (n=40) and associated with poor overall survival. Our data suggest that RET activating mutations confers resistance to chemotherapeutic agents by preventing apoptosis through inhibition of ATF4 activity and repression of proapoptotic gene Noxa. These findings reveal the importance of nuclear localization of RET, as newly discovered modulator of nuclear gene expression. This newly identified interaction between RET kinase and ATF4 has important implication for MTC and other RET/ATF4-mediated cancer types. Since ATF4 transcription is induced by endoplasmic reticulum stress, and activation of ATF4 induces cell cycle arrest and apoptosis, the concept of promoting endoplasmic reticulum stress in combination with tyrosine kinase inhibitors could be considered as a therapeutic strategy for cancer. Citation Format: Rozita Bagheri-Yarmand, Anupama E Gururaj, Michelle Williams, Zamal Ahmed, Jean E Ladbury, Oliver Bogler, Sue-chen Huang, Gilbert G Cote, Robert F Gagel. RET tyrosine kinase receptor represses Noxa transcription and prevents genotoxic or endoplasmic reticulum stress-induced apoptosis. [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 1730. doi:10.1158/1538-7445.AM2013-1730

Abstract #5032 Cyclin E is a G (1) cyclin essential for S-phase entry and has a profound role in oncogenesis. Cyclin E is overexpressed and present in low-molecular-weight (LMW) isoforms in breast cancer cells and tumor tissues compared to normal cells and tissues. Such alteration of cyclin E is linked to poor patient outcome. We have previously shown that ectopic expression of low molecular weight of Cyclin E (LMW-E) in breast cancer cell lines induces genomic instability by promoting aneuploidy and the formation of structural chromosomal lesion. To determine the mechanism of induction of genomic instability by ectopic expression of LMW-E, we generated the MCF-7 cells inducibly expressing either the full length (EL) or low molecular weight (LMW-E) form of cyclin E. We observed 5.5 fold higher number of cells with centrosome defects in LMW-E induced versus EL induced cells. Notably, elevated levels of LMW-E led to a significant increase in fragmented nuclei and multinucleated cells. Immunofluorescence analysis also showed a 4-5 fold increase in the number of cells containing lagging chromosomes and anaphase bridges. Karyotype analysis for gross chromosomal aberrations metaphase preparation of the uninduced and EL and LMW induced cells showed a significantly higher percentage of chromosomal defects including dicentric chromosome and chromosomal breaks associated with LMW-E overexpression. We also show that while overexpression of EL leads to accumulation of cells in S and G2/M phases of the cell cycle, the overexpression of LMW of cyclin E causes a faster M phase progression. As a consequence, LMW-cyclin E overexpressing cells are resistant to nocodazole-induced M phase arrest. Using biochemical assays, we show that EL and LMW-E interact with the phosphatase Cdc25C and when either EL or LMW are in complex with CDK2 that they phosphorylate Cdc25C. Such association of LMW-E with Cdc25C results premature inactivation of cyclin B/CDK1 upon treatment of these cells with nocodazole. Additionally, continued induction of LMW-E results in premature inactivation of PLK1 in mitotic cells and deregulated localization of PLK1 such that it is no longer associated to central spindles during anaphase and telophase. Collectively, these results suggest that overexpression of LMW-E interferes with the activation of CDK1 during mitosis. The relatively low activity of CDK1 during the passage through prophase, prometaphase, and metaphase interferes with the regulation of the spindle checkpoint and facilitates the premature transition of dividing cells from metaphase to anaphase resulting in chromosomal instability and aneuploidy. This is the first observation of how mechanistically the overexpression of a proto-oncogene can lead to genomic instability in breast cancer. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5032.