Overexpression of CD36 in mammary fibroblasts suppresses colony growth in breast cancer cell lines

Human breast tumors are not fully autonomous. They are dependent on nutrients and growth-promoting signals provided by the supporting stromal cells. Within the tumor microenvironment, one of the secreted macromolecules by tumor cells is activin A, where we show to downregulate CD36 in fibroblasts. Downregulation of CD36 in fibroblasts also increases the secretion of activin A by fibroblasts. We hypothesize that overexpression of CD36 in fibroblasts inhibits the formation of solid tumors in subtypes of breast cancer models. For the first time, we show that co-culturing organoid models of breast cancer cell lines of MDA-MB-231 (e.g., a triple-negative line) or MCF7 (e.g., a luminal-A line) with CD36⁺ fibroblasts inhibit the growth and normalizes basal and lateral polarities, respectively. In the long-term anchorage-independent growth assay, the rate of colony formation is also reduced for MDA-MB-231. These observations are consistent with the mechanism of tumor suppression involving the downregulation of pSMAD2/3 and YY1 expression levels. Our integrated analytical methods leverage and extend quantitative assays at cell- and colony-scales in both short- and long-term cultures using brightfield or immunofluorescent microscopy and robust image analysis. Conditioned media are profiled with the ELISA assay.

Propagation of functional estrogen receptor positive normal human breast cells in 3D cultures

PURPOSE

Understanding how differentiation, microenvironment, and hormonal milieu influence human breast cell susceptibility to malignant transformation will require the use of physiologically relevant in vitro systems. We sought to develop a 3D culture model that enables the propagation of normal estrogen receptor alpha (ER) + cells.

METHODS

We tested soluble factors and protocols for the ability to maintain progenitor and ER + cells in cultures established from primary cells. Optimized conditions were then used to profile estrogen-induced gene expression changes in cultures from three pathology-free individuals.

RESULTS

Long-term representation of ER + cells was optimal in medium that included three different TGFβ/activin receptor-like kinase inhibitors. We found that omitting the BMP signaling antagonist, Noggin, enhanced the responsiveness of the PGR gene to estradiol exposure without altering the proportions of ER + cells in the cultures. Profiling of estradiol-exposed cultures showed that while all the cultures showed immediate and robust induction of PGR, LRP2, and IGFB4, other responses varied qualitatively and quantitatively across specimens.

CONCLUSIONS

We successfully identified conditions for the maintenance and propagation of functional ER + cells from normal human breast tissues. We propose that these 3D cultures will overcome limitations of conventional 2D cultures of partially or fully transformed cell lines by sustaining normal endocrine function and growth regulation of the cell populations that comprise intact breasts.

Genome-Wide Overexpression Screen Identifies Genes Able to Bypass p16-Mediated Senescence in Melanoma

Malignant melanomas often arise from nevi, which result from initial oncogene-induced hyperproliferation of melanocytes that are maintained in a CDKN2A/p16-mediated senescent state. Thus, genes that can bypass this senescence barrier are likely to contribute to melanoma development. We have performed a gain-of-function screen of 17,030 lentivirally expressed human open reading frames (ORFs) in a melanoma cell line containing an inducible p16 construct to identify such genes. Genes known to bypass p16-induced senescence arrest, including the human papilloma virus 18 E7 gene ( HPV18E7), and genes such as the p16-binding CDK6 with expected functions, as well as panel of novel genes, were identified, including high-mobility group box (HMGB) proteins. A number of these were further validated in two other models of p16-induced senescence. Tissue immunohistochemistry demonstrated higher levels of CDK6 in primary melanomas compared with normal skin and nevi. Reduction of CDK6 levels drove melanoma cells expressing functional p16 into senescence, demonstrating its contribution to bypass senescence.

Designing a broad-spectrum integrative approach for cancer prevention and treatment

Targeted therapies and the consequent adoption of "personalized" oncology have achieved notable successes in some cancers; however, significant problems remain with this approach. Many targeted therapies are highly toxic, costs are extremely high, and most patients experience relapse after a few disease-free months. Relapses arise from genetic heterogeneity in tumors, which harbor therapy-resistant immortalized cells that have adopted alternate and compensatory pathways (i.e., pathways that are not reliant upon the same mechanisms as those which have been targeted). To address these limitations, an international task force of 180 scientists was assembled to explore the concept of a low-toxicity "broad-spectrum" therapeutic approach that could simultaneously target many key pathways and mechanisms. Using cancer hallmark phenotypes and the tumor microenvironment to account for the various aspects of relevant cancer biology, interdisciplinary teams reviewed each hallmark area and nominated a wide range of high-priority targets (74 in total) that could be modified to improve patient outcomes. For these targets, corresponding low-toxicity therapeutic approaches were then suggested, many of which were phytochemicals. Proposed actions on each target and all of the approaches were further reviewed for known effects on other hallmark areas and the tumor microenvironment. Potential contrary or procarcinogenic effects were found for 3.9% of the relationships between targets and hallmarks, and mixed evidence of complementary and contrary relationships was found for 7.1%. Approximately 67% of the relationships revealed potentially complementary effects, and the remainder had no known relationship. Among the approaches, 1.1% had contrary, 2.8% had mixed and 62.1% had complementary relationships. These results suggest that a broad-spectrum approach should be feasible from a safety standpoint. This novel approach has potential to be relatively inexpensive, it should help us address stages and types of cancer that lack conventional treatment, and it may reduce relapse risks. A proposed agenda for future research is offered.

Parabens and Human Epidermal Growth Factor Receptor Ligand Cross-Talk in Breast Cancer Cells

BACKGROUND

Xenoestrogens are synthetic compounds that mimic endogenous estrogens by binding to and activating estrogen receptors. Exposure to estrogens and to some xenoestrogens has been associated with cell proliferation and an increased risk of breast cancer. Despite evidence of estrogenicity, parabens are among the most widely used xenoestrogens in cosmetics and personal-care products and are generally considered safe. However, previous cell-based studies with parabens do not take into account the signaling cross-talk between estrogen receptor α (ERα) and the human epidermal growth factor receptor (HER) family.

OBJECTIVES

We investigated the hypothesis that the potency of parabens can be increased with HER ligands, such as heregulin (HRG).

METHODS

The effects of HER ligands on paraben activation of c-Myc expression and cell proliferation were determined by real-time polymerase chain reaction, Western blots, flow cytometry, and chromatin immunoprecipitation assays in ERα- and HER2-positive human BT-474 breast cancer cells.

RESULTS

Butylparaben (BP) and HRG produced a synergistic increase in c-Myc mRNA and protein levels in BT-474 cells. Estrogen receptor antagonists blocked the synergistic increase in c-Myc protein levels. The combination of BP and HRG also stimulated proliferation of BT-474 cells compared with the effects of BP alone. HRG decreased the dose required for BP-mediated stimulation of c-Myc mRNA expression and cell proliferation. HRG caused the phosphorylation of serine 167 in ERα. BP and HRG produced a synergistic increase in ERα recruitment to the c-Myc gene.

CONCLUSION

Our results show that HER ligands enhanced the potency of BP to stimulate oncogene expression and breast cancer cell proliferation in vitro via ERα, suggesting that parabens might be active at exposure levels not previously considered toxicologically relevant from studies testing their effects in isolation.

CITATION

Pan S, Yuan C, Tagmount A, Rudel RA, Ackerman JM, Yaswen P, Vulpe CD, Leitman DC. 2016. Parabens and human epidermal growth factor receptor ligand cross-talk in breast cancer cells. Environ Health Perspect 124:563-569; http://dx.doi.org/10.1289/ehp.1409200.

Abstract B34: PIM kinase as a novel therapeutic target for triple-negative breast cancer

The greatest clinical challenge in treating breast cancer occurs in those patients whose tumors lack expression of the estrogen and progesterone receptors and that of the HER2 oncoprotein. No targeted therapeutic strategies currently exist against this aggressive type of “triple negative” breast cancer (TNBC) due to lack of validated targets. We previously found that MYC mRNA, protein, and its signaling were disproportionally elevated in TN compared to receptor positive (RP) breast cancer. We sought to take advantage of the unique molecular feature found in this tumor type to identify potent and effective treatment strategies. Since MYC is an oncogenic transcription factor, rationally designed small molecule inhibitors that can directly inhibit its activity are not available for clinical use. An alternative approach to selectively kill MYC-driven tumors is to inhibit those proteins that are indispensable for the viability of such tumors, but are not essential in non-tumorigenic cells. This form of “indirect” treatment strategy has become known as the “synthetic-lethal” approach. To identify novel targets that are readily druggable for treating MYC-driven TNBC, we conducted a kinome MYC synthetic lethal shRNA screen in non-immortalized human mammary epithelial cells expressing a 4-hydroxytamoxifen (TAM)-activatable MycER transgene (HMEC-MycER). Of 600 human kinases targeted by 2,000 individual shRNA clones, 9 kinases were identified as hits as they were essential specifically for the MYC-activated HMEC cells. Among these hits, we focused on PIM1, a non-essential kinase, the knock-down of which had the greatest efficacy in causing cell death in the MYC-activated cells and had minimum inhibitory effect on the growth of the control cells. We determined that PIM1 expression was elevated in TN tumors and was associated with poor prognosis specifically in patients with hormone receptor-negative tumors. Small molecule PIM kinase inhibitors halted the growth of human TN tumors with elevated MYC expression in a patient-derived tumor xenograft (PDX) mouse model by inhibiting oncogenic transcriptional activity of MYC while simultaneously restoring the function of the endogenous cell cycle inhibitor p27. Thus, our findings warrant clinical evaluation of small molecule PIM kinase inhibitors in patients with TN tumors that exhibit elevated MYC expression.

Note: This abstract was not presented at the conference.

Citation Format: Dai Horiuchi, Alicia Y. Zhou, Alexandra N. Corella, Christina Yau, Sanjeev Balakrishnan, Kai Kessenbrock, Devon A. Lawson, Roman Camarda, Brittany N. Anderton, Alexey V. Bazarov, Henok Eyob, Julia Rohrberg, Paul Yaswen, Michael T. McManus, Hope S. Rugo, Zena Werb, Andrei Goga. PIM kinase as a novel therapeutic target for triple-negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B34.

A TORC2-Akt Feed-Forward Topology Underlies HER3 Resiliency in HER2-Amplified Cancers

The requisite role of HER3 in HER2-amplified cancers is beyond what would be expected as a dimerization partner or effector substrate and it exhibits a substantial degree of resiliency that mitigates the effects of HER2-inhibitor therapies. To better understand the roots of this resiliency, we conducted an in-depth chemical-genetic interrogation of the signaling network downstream of HER3. A unique attribute of these tumors is the deregulation of TORC2. The upstream signals that ordinarily maintain TORC2 signaling are lost in these tumors, and instead TORC2 is driven by Akt. We find that in these cancers HER3 functions as a buffering arm of an Akt-TORC2 feed-forward loop that functions as a self-perpetuating module. This network topology alters the role of HER3 from a conditionally engaged ligand-driven upstream physiologic signaling input to an essential component of a concentric signaling throughput highly competent at preservation of homeostasis. The competence of this signaling topology is evident in its response to perturbation at any of its nodes. Thus, a critical pathophysiologic event in the evolution of HER2-amplified cancers is the loss of the input signals that normally drive TORC2 signaling, repositioning it under Akt dependency, and fundamentally altering the role of HER3. This reprogramming of the downstream network topology is a key aspect in the pathogenesis of HER2-amplified cancers and constitutes a formidable barrier in the targeted therapy of these cancers.

Single-cell analysis reveals a stem-cell program in human metastatic breast cancer cells

Despite major advances in understanding the molecular and genetic basis of cancer, metastasis remains the cause of >90% of cancer-related mortality. Understanding metastasis initiation and progression is critical to developing new therapeutic strategies to treat and prevent metastatic disease. Prevailing theories hypothesize that metastases are seeded by rare tumour cells with unique properties, which may function like stem cells in their ability to initiate and propagate metastatic tumours. However, the identity of metastasis-initiating cells in human breast cancer remains elusive, and whether metastases are hierarchically organized is unknown. Here we show at the single-cell level that early stage metastatic cells possess a distinct stem-like gene expression signature. To identify and isolate metastatic cells from patient-derived xenograft models of human breast cancer, we developed a highly sensitive fluorescence-activated cell sorting (FACS)-based assay, which allowed us to enumerate metastatic cells in mouse peripheral tissues. We compared gene signatures in metastatic cells from tissues with low versus high metastatic burden. Metastatic cells from low-burden tissues were distinct owing to their increased expression of stem cell, epithelial-to-mesenchymal transition, pro-survival, and dormancy-associated genes. By contrast, metastatic cells from high-burden tissues were similar to primary tumour cells, which were more heterogeneous and expressed higher levels of luminal differentiation genes. Transplantation of stem-like metastatic cells from low-burden tissues showed that they have considerable tumour-initiating capacity, and can differentiate to produce luminal-like cancer cells. Progression to high metastatic burden was associated with increased proliferation and MYC expression, which could be attenuated by treatment with cyclin-dependent kinase (CDK) inhibitors. These findings support a hierarchical model for metastasis, in which metastases are initiated by stem-like cells that proliferate and differentiate to produce advanced metastatic disease.

Reinforcing targeted therapeutics with phenotypic stability factors

Deregulated cell cycle progression can often be traced to intrinsic defects in specific regulatory proteins in cancer cells. Knowledge of these primary defects has led to targeted approaches that exploit the defects and spare normal cells. However, the success of such targeted approaches is still hit-or-miss. Genetic and epigenetic variability inherent in most tumors often results in phenotypic heterogeneity that, in turn, results in de novo or acquired resistance to therapeutic agents. The ability of cells to compensate and adapt to the inhibition of a specific cell cycle mediator is not remarkable. What is novel and of great potential importance is that the ability of cells to exhibit such adaptability varies markedly. "Phenotypic stability factors" that restrict the ability of cells to undergo epithelial-mesenchymal transitions (EMT) may dictate the success or failure of targeted therapies by interfering with compensatory changes such as deregulation of CDK2 activity. Identification of existing and new agents that induce and maintain phenotypic stability factors will inform and enable synergistic approaches to the eradication of even the most aggressive tumors.

Modelling breast cancer requires identification and correction of a critical cell lineage-dependent transduction bias

Clinically relevant human culture models are essential for developing effective therapies and exploring the biology and etiology of human cancers. Current breast tumour models, such as those from oncogenically transformed primary breast cells, produce predominantly basal-like properties, whereas the more common phenotype expressed by the vast majority of breast tumours are luminal. Reasons for this puzzling, yet important phenomenon, are not understood. We show here that luminal epithelial cells are significantly more resistant to viral transduction than their myoepithelial counterparts. We suggest that this is a significant barrier to generating luminal cell lines and experimental tumours in vivo and to accurate interpretation of results. We show that the resistance is due to lower affinity of luminal cells for virus attachment, which can be overcome by pretreating cells—or virus—with neuraminidase. We present an analytical method for quantifying transductional differences between cell types and an optimized protocol for transducing unsorted primary human breast cells in context.

Clinical breast cancers predominantly present luminal features, but experimental models are essentially basal. Here the authors show that luminal cells are significantly less susceptible to viral transduction, and present methods to analyse and overcome the bias in heterogeneous populations.

Therapeutic targeting of replicative immortality

One of the hallmarks of malignant cell populations is the ability to undergo continuous proliferation. This property allows clonal lineages to acquire sequential aberrations that can fuel increasingly autonomous growth, invasiveness, and therapeutic resistance. Innate cellular mechanisms have evolved to regulate replicative potential as a hedge against malignant progression. When activated in the absence of normal terminal differentiation cues, these mechanisms can result in a state of persistent cytostasis. This state, termed “senescence,” can be triggered by intrinsic cellular processes such as telomere dysfunction and oncogene expression, and by exogenous factors such as DNA damaging agents or oxidative environments. Despite differences in upstream signaling, senescence often involves convergent interdependent activation of tumor suppressors p53 and p16/pRB, but can be induced, albeit with reduced sensitivity, when these suppressors are compromised. Doses of conventional genotoxic drugs required to achieve cancer cell senescence are often much lower than doses required to achieve outright cell death. Additional therapies, such as those targeting cyclin dependent kinases or components of the PI3K signaling pathway, may induce senescence specifically in cancer cells by circumventing defects in tumor suppressor pathways or exploiting cancer cells’ heightened requirements for telomerase. Such treatments sufficient to induce cancer cell senescence could provide increased patient survival with fewer and less severe side effects than conventional cytotoxic regimens. This positive aspect is countered by important caveats regarding senescence reversibility, genomic instability, and paracrine effects that may increase heterogeneity and adaptive resistance of surviving cancer cells. Nevertheless, agents that effectively disrupt replicative immortality will likely be valuable components of new combinatorial approaches to cancer therapy.

Abstract LB-122: PIM1 kinase inhibition halts the growth of MYC-overexpressing triple-negative breast tumors

The proto-oncogene MYC affects diverse cellular processes ranging from cell proliferation, metabolism, to pluripotency of embryonic stem cells. While de-regulated MYC signaling has been commonly identified in a variety of human malignancies, no therapeutic strategies have been clinically established to specifically target tumors that present elevated MYC expression. We have previously reported that triple-negative (TN) breast cancers, which lack the expression of predictive biomarkers of response (i.e., estrogen/progesterone receptors, human epidermal growth factor receptor 2), exhibit significantly elevated MYC expression as well as MYC pathway activation (Horiuchi, D., et al. 2012 JEM). To understand the vulnerabilities of TN tumors, we performed a kinome synthetic-lethal shRNA screen in human mammary epithelial cells in which MYC activity can be controlled (i.e., HMEC-MycER cells). The screen yielded 10+ potential synthetic lethal partners of MYC, including recently described AMPK-related kinase 5 (Liu, L., et al. 2012. Nature). One of our top hits was the proto-oncogene PIM1, a non-essential serine/threonine kinase previously shown to be a genetic enhancer of MYC in transgenic mouse models of lymphomas and prostate cancer. Our bioinformatics studies performed on four independent clinical cohorts (n: 146 ∼ 683 patients each) showed that in all cohorts both MYC and PIM1 were significantly enriched in the TN populations. PIM1 expression was correlated with that of MYC in three out of the four cohorts. Furthermore, we found that PIM1 overexpression alone was a poor prognostic factor associated with diminished recurrence-free survival. Our in vivo efficacy studies using conventional as well as novel “human-in-mouse,” patient derived orthotopic xenograft (PDX) models demonstrated that PIM was essential for the growth of MYC-overexpressing human TN tumors. In the PDX tumors, PIM inhibition nearly completely arrested tumor growth, which was associated with significant up-regulation of an endogenous cell cycle inhibitor p27, a known PIM1 substrate, down-regulation of protein synthesis machinery, and inhibition of MYC activity itself. These observations held true in a panel of breast cancer cell lines, and elevating the p27 expression alone was sufficient to inhibit cancer cell proliferation. Thus, our findings warrant the use of PIM kinase inhibitors in treating TN tumors that exhibit the elevated expression of MYC and/or PIM.

Citation Format: Dai Horiuchi, Alicia Y. Zhou, Alexandra N. Corella, Christina Yau, Devon A. Lawson, Alexey V. Bazarov, Paul Yaswen, Michael T. McManus, Zena Werb, Alana L. Welm, Andrei Goga. PIM1 kinase inhibition halts the growth of MYC-overexpressing triple-negative breast tumors. [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 LB-122. doi:10.1158/1538-7445.AM2014-LB-122

Abstract B232: PIM1 kinase is essential for the growth of MYC-overexpressing triple-negative breast tumors and is an efficacious therapeutic target

While de-regulated MYC signaling has been identified in a wide variety of human malignancies, no targeted therapeutic strategies have been established to target tumors with elevated MYC expression. We have previously reported that receptor “triple negative” (TN) breast cancers, which lack predictive biomarkers of response (i.e., estrogen/progesterone receptors, human epidermal growth factor receptor 2), exhibit significantly elevated MYC expression as well as MYC pathway activation (Horiuchi, D., et al. 2012 JEM). In an effort to uncover the potential Achilles’ heels of TN tumors, we performed a kinome, synthetic-lethal shRNA screen in human mammary epithelial cells in which MYC activity can be controlled (i.e., HMEC-MycER cells). The screen yielded 13 potential synthetic lethal partners of MYC, including recently described AMPK-related kinase 5 (Liu, L., et al. 2012. Nature). One of our top hits is the proto-oncogene PIM1, previously shown by others to be a genetic enhancer of MYC in transgenic mouse models of lymphomas and prostate cancer. Our bioinformatics studies performed on four independent clinical cohorts (n: 146 ∼ 683 patients each) show that in all the cohorts both MYC and PIM1 are significantly enriched in the TN populations. MYC and PIM1 expression are correlated in three out of the four cohorts. Furthermore, we find that PIM1 overexpression is a poor prognostic factor associated with diminished recurrence-free survival. Our in vivo efficacy studies using conventional as well as novel “human-in-mouse” orthotopic xenograft models demonstrate that PIM1 is required for the growth of MYC-overexpressing human TN tumors. We are currently investigating the cellular mechanisms by which PIM kinase inhibition causes such growth defects.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B232.

Citation Format: Dai Horiuchi, Alicia Y. Zhou, Alexandra N. Corella, Christina Yau, Devon A. Lawson, Alexey V. Bazarov, Paul Yaswen, Michael T. McManus, Alana L. Welm, Zena Werb, Andrei Goga. PIM1 kinase is essential for the growth of MYC-overexpressing triple-negative breast tumors and is an efficacious therapeutic target. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B232.

Abstract 3043: Akt-mTorc2 signaling underlies cellular addiction to HER3 signaling in HER2-amplified cancers

HER2 amplified breast cancers are characterized by constitutive HER3 phosphorylation. Efforts to target the functionally relevant HER2-HER3 signaling complex in these tumor cells has proven difficult due to robust feedback signaling mechanisms that restore its signaling in the face of anything short of the complete inactivation of HER2 kinase activity. To study this in more depth, we used a combination of genetic and chemical genetic approaches designed to study the cellular adaptational responses to network perturbation. We find that positive or negative perturbation of any signaling node from upstream HER2 or HER3 down to the two mTor complexes results in compensatory responses that function to maintain homeostasis. The responses to various perturbations are mechanistically distinct, but taken together reveal an uncompromising core topology driven by a concentric Akt-mTorc2 positive-feedback loop buffered by HER3 and PHLPP nodes. While mTorc1 signaling is also subject to feedback regulation, it is not involved in the core topology and its output is not always protected in these tumors. The central role of mTor is consistent with its known functions as a master regulator of cellular homeostasis highly conserved across eukaryotes. But the inclusion of HER3 in this central homeostatic network is a tumor cell anomaly. To better delineate how this homeostatic network is altered in tumor cells, we compared and contrasted the network topology in a panel of HER2-amplified cancer cells with a panel of immortalized breast epithelial cells. Much of the feedback circuitries are preserved across tumor and untransformed cells. But the driving Akt-mTorc2 positive-feedback loop is lacking in untransformed cells. This is due to the fact that Akt drives mTorc2 signaling in tumor cells but not in untransformed cells. Consistent with a central role of mTorc2 in these tumors, knockdown of Rictor, but not Raptor, is lethal in HER2-amplified SkBr3 tumor cells. These studies reveal a key alteration of Akt-mTor signaling in HER2 driven tumors that underlies the pathological incorporation of HER3 into an mTor-driven network, thus putting the HER2-HER3 tumor driver under the protection of this evolutionarily ancient pathway, highly competent at homeostasis.

Citation Format: Dhara N. Amin, Deepika Ahuja, Gregory S. Ducker, Paul Yaswen, Kevan Shokat, Mark Moasser. Akt-mTorc2 signaling underlies cellular addiction to HER3 signaling in HER2-amplified cancers. [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 3043. doi:10.1158/1538-7445.AM2013-3043

ECM microenvironment regulates collective migration and local dissemination in normal and malignant mammary epithelium

Breast cancer progression involves genetic changes and changes in the extracellular matrix (ECM). To test the importance of the ECM in tumor cell dissemination, we cultured epithelium from primary human breast carcinomas in different ECM gels. We used basement membrane gels to model the normal microenvironment and collagen I to model the stromal ECM. In basement membrane gels, malignant epithelium either was indolent or grew collectively, without protrusions. In collagen I, epithelium from the same tumor invaded with protrusions and disseminated cells. Importantly, collagen I induced a similar initial response of protrusions and dissemination in both normal and malignant mammary epithelium. However, dissemination of normal cells into collagen I was transient and ceased as laminin 111 localized to the basal surface, whereas dissemination of carcinoma cells was sustained throughout culture, and laminin 111 was not detected. Despite the large impact of ECM on migration strategy, transcriptome analysis of our 3D cultures revealed few ECM-dependent changes in RNA expression. However, we observed many differences between normal and malignant epithelium, including reduced expression of cell-adhesion genes in tumors. Therefore, we tested whether deletion of an adhesion gene could induce sustained dissemination of nontransformed cells into collagen I. We found that deletion of P-cadherin was sufficient for sustained dissemination, but exclusively into collagen I. Our data reveal that metastatic tumors preferentially disseminate in specific ECM microenvironments. Furthermore, these data suggest that breaks in the basement membrane could induce invasion and dissemination via the resulting direct contact between cancer cells and collagen I.

Age-associated gene expression in normal breast tissue mirrors qualitative age-at-incidence patterns for breast cancer

BACKGROUND

Age is the strongest breast cancer risk factor, with overall breast cancer risk increasing steadily beginning at approximately 30 years of age. However, while breast cancer risk is lower among younger women, young women's breast cancer may be more aggressive. Although, several genomic and epidemiologic studies have shown higher prevalence of aggressive, estrogen-receptor negative breast cancer in younger women, the age-related gene expression that predisposes to these tumors is poorly understood. Characterizing age-related patterns of gene expression in normal breast tissues may provide insights on etiology of distinct breast cancer subtypes that arise from these tissues.

METHODS

To identify age-related changes in normal breast tissue, 96 tissue specimens from patients with reduction mammoplasty, ages 14 to 70 years, were assayed by gene expression microarray.

RESULTS

Significant associations between gene expression levels and age were identified for 802 probes (481 increased, 321 decreased with increasing age). Enriched functions included "aging of cells," "shape change," and "chemotaxis," and enriched pathways included Wnt/beta-catenin signaling, Ephrin receptor signaling, and JAK/Stat signaling. Applying the age-associated genes to publicly available tumor datasets, the age-associated pathways defined two groups of tumors with distinct survival.

CONCLUSION

The hazard rates of young-like tumors mirrored that of high-grade tumors in the Surveillance, Epidemiology, and End Results Program, providing a biologic link between normal aging and age-related tumor aggressiveness.

IMPACT

These data show that studies of normal tissue gene expression can yield important insights about the pathways and biologic pressures that are relevant during tumor etiology and progression.

The transcription factor ZNF217 is a prognostic biomarker and therapeutic target during breast cancer progression

The transcription factor ZNF217 is a candidate oncogene in the amplicon on chromosome 20q13 that occurs in 20% to 30% of primary human breast cancers and that correlates with poor prognosis. We show that Znf217 overexpression drives aberrant differentiation and signaling events, promotes increased self-renewal capacity, mesenchymal marker expression, motility, and metastasis, and represses an adult tissue stem cell gene signature downregulated in cancers. By in silico screening, we identified candidate therapeutics that at low concentrations inhibit growth of cancer cells expressing high ZNF217. We show that the nucleoside analogue triciribine inhibits ZNF217-induced tumor growth and chemotherapy resistance and inhibits signaling events [e.g., phospho-AKT, phospho-mitogen-activated protein kinase (MAPK)] in vivo. Our data suggest that ZNF217 is a biomarker of poor prognosis and a therapeutic target in patients with breast cancer and that triciribine may be part of a personalized treatment strategy in patients overexpressing ZNF217. Because ZNF217 is amplified in numerous cancers, these results have implications for other cancers.

SIGNIFICANCE

This study finds that ZNF217 is a poor prognostic indicator and therapeutic target in patients with breast cancer and may be a strong biomarker of triciribine treatment efficacy in patients. Because previous clinical trials for triciribine did not include biomarkers of treatment efficacy, this study provides a rationale for revisiting triciribine in the clinical setting as a therapy for patients with breast cancer who overexpress ZNF217.

Abstract 1680: Age-associated gene expression in normal breast tissue provide insights regarding associations between aging and aggressive tumor characteristics

Age is the strongest breast cancer risk factor, with overall breast cancer risk increasing steadily beginning at approximately 30 years of age. Though several genomic studies have characterized differences in the biology of tumors arising in younger vs. older women, understanding of age-related gene expression in the non-malignant tissue is lacking. Because non-malignant tissue represents the microenvironment and/or field from which the tumor arises, an improved characterization of the age-related change in normal breast may provide suggestions of important pathways in the etiology of distinct breast cancer subtypes. To identify age-related changes in the breast microenvironment, 96 normal tissue specimens from reduction mammoplasty patients ranging in age from 14 to 70 were assayed by gene expression microarray. Significant associations between gene expression levels and age were identified for 802 genes (481 increased and 321 decreased with increasing age). Enriched functions included ‘aging of cells’, ‘shape change’, and ‘chemotaxis’, and enriched pathways included Wnt/beta-catenin signaling, Ephrin Receptor Signaling, Oncostatin M signaling, and JAK/Stat Signaling. Applying the age-associated genes to publicly available tumor datasets, the age-associated pathways defined two groups of tumors with distinct survival. The poor prognosis tumor group shared features of ‘younger’ breast tissue gene expression, providing a biological link to qualitative patterns in age at incidence. Aggressive tumors common in younger women (such as ER negative and basal-like breast cancers) harbor gene expression patterns that reflect the normal tissue from which they arise. For the young, these signatures portend more aggressive disease. These data show that studies of normal tissue gene expression can yield important insights about the pathways and biological selection factors that are relevant during tumor progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1680. doi:1538-7445.AM2012-1680

MYC pathway activation in triple-negative breast cancer is synthetic lethal with CDK inhibition

Triple-negative breast cancers with elevated MYC are sensitized to CDK inhibition.

Estrogen, progesterone, and HER2 receptor-negative triple-negative breast cancers encompass the most clinically challenging subtype for which targeted therapeutics are lacking. We find that triple-negative tumors exhibit elevated MYC expression, as well as altered expression of MYC regulatory genes, resulting in increased activity of the MYC pathway. In primary breast tumors, MYC signaling did not predict response to neoadjuvant chemotherapy but was associated with poor prognosis. We exploit the increased MYC expression found in triple-negative breast cancers by using a synthetic-lethal approach dependent on cyclin-dependent kinase (CDK) inhibition. CDK inhibition effectively induced tumor regression in triple-negative tumor xenografts. The proapoptotic BCL-2 family member BIM is up-regulated after CDK inhibition and contributes to this synthetic-lethal mechanism. These results indicate that aggressive breast tumors with elevated MYC are uniquely sensitive to CDK inhibitors.

The specific role of pRb in p16 (INK4A) -mediated arrest of normal and malignant human breast cells

RB family proteins pRb, p107 and p130 have similar structures and overlapping functions, enabling cell cycle arrest and cellular senescence. pRb, but not p107 or p130, is frequently mutated in human malignancies. In human fibroblasts acutely exposed to oncogenic ras, pRb has a specific role in suppressing DNA replication, and p107 or p130 cannot compensate for the loss of this function; however, a second p53/p21-dependent checkpoint prevents escape from growth arrest. This model of oncogene-induced senescence requires the additional loss of p53/p21 to explain selection for preferential loss of pRb function in human malignancies. We asked whether similar rules apply to the role of pRb in growth arrest of human epithelial cells, the source of most cancers. In two malignant human breast cancer cell lines, we found that individual RB family proteins were sufficient for the establishment of p16-initiated senescence, and that growth arrest in G 1 was not dependent on the presence of functional pRb or p53. However, senescence induction by endogenous p16 was delayed in primary normal human mammary epithelial cells with reduced pRb but not with reduced p107 or p130. Thus, under these circumstances, despite the presence of functional p53, p107 and p130 were unable to completely compensate for pRb in mediating senescence induction. We propose that early inactivation of pRb in pre-malignant breast cells can, by itself, extend proliferative lifespan, allowing acquisition of additional changes necessary for malignant transformation.

Who is in the driver's seat in 8p12 amplifications? ZNF703 in luminal B breast tumors

Two recent reports identify ZNF703 as an oncogene driving selection of frequent chromosome 8p12 amplifications in luminal B breast tumors. The estrogen-responsive ZNF703 gene encodes a transcriptional cofactor that, when overexpressed, induces cell proliferation and interferes with transforming growth factor beta signaling. In MCF7 cells, increased ZNF703 expression results in activation of genes involved in stem cell self-renewal - while in primary human mammary epithelial cells, ZNF703 increases the ratio of luminal to basal progenitors. Expression of the murine homolog of ZNF703 reduces cell adhesion and promotes metastasis. ZNF703 overexpression thus alters regulation of proliferation and differentiation in luminal B tumors.

Raf-induced MMP9 disrupts tissue architecture of human breast cells in three-dimensional culture and is necessary for tumor growth in vivo

Organization into polarized three-dimensional structures defines whether epithelial cells are normal or malignant. In a model of morphogenesis, we show that inhibiting key signaling pathways in human breast cancer cells leads to "phenotypic reversion" of the malignant cells. Using architecture as an endpoint, we report that, in all cases, signaling through Raf/MEK/ERK disrupted tissue polarity via matrix metalloproteinase9 (MMP9) activity. Induction of Raf or activation of an engineered, functionally inducible MMP9 in nonmalignant cells led to loss of tissue polarity, and reinitiated proliferation. Conversely, inhibition of Raf or MMP9 with small molecule inhibitors or shRNAs restored the ability of cancer cells to form polarized quiescent structures. Silencing MMP9 expression also reduced tumor growth dramatically in a murine xenograft model. LC-MS/MS analysis comparing conditioned medium from nonmalignant cells with or without active MMP9 revealed laminin 111 (LM1) as an important target of MMP9. LM1 has been implicated in acinar morphogenesis; thus, its degradation by MMP9 provides a mechanism for loss of tissue polarity and reinitiation of growth associated with MMP9 activity. These findings underscore the importance of the dynamic reciprocity between the extracellular matrix integrity, tissue polarity, and Raf/MEK/ERK and MMP9 activities, providing an axis for either tissue homeostasis or malignant progression.

p16(INK4a) -mediated suppression of telomerase in normal and malignant human breast cells

The cyclin-dependent kinase inhibitor p16(INK4a) (CDKN2A) is an important tumor suppressor gene frequently inactivated in human tumors. p16 suppresses the development of cancer by triggering an irreversible arrest of cell proliferation termed cellular senescence. Here, we describe another anti-oncogenic function of p16 in addition to its ability to halt cell cycle progression. We show that transient expression of p16 stably represses the hTERT gene, encoding the catalytic subunit of telomerase, in both normal and malignant breast epithelial cells. Short-term p16 expression increases the amount of histone H3 trimethylated on lysine 27 (H3K27) bound to the hTERT promoter, resulting in transcriptional silencing, likely mediated by polycomb complexes. Our results indicate that transient p16 exposure may prevent malignant progression in dividing cells by irreversible repression of genes, such as hTERT, whose activity is necessary for extensive self-renewal.

Abstract 3210: pRb, but not p107 or p130, is required for full induction of p16INK4a in human mammary epithelial cells

Pocket family proteins pRb, p107 and p130 are important cell cycle regulators in mammalian cells. In addition to controlling G1 to S transition, they also participate in differentiation and senescence. While all three pocket proteins have similar structures and apparently overlapping functions, defects in only one, pRb, are associated with human cancers. In breast cancer, pRb loss is associated with triple negative (ER-, PR-, HER2-) tumors, which tend to be more aggressive. To explore possible functions of pRb that may be unique, we investigated relative contributions of pocket proteins to cellular senescence initiated by high levels of the cyclin-dependent kinase inhibitor p16INK4a. For this purpose, we silenced pRb, p107 and p130 expression individually with shRNA constructs and exposed MCF7 and MDA-MB-231 breast cancer cell lines to tet-inducible p16. In both malignant lines, knockdown of any single pocket protein was insufficient to prevent p16-induced senescence, indicating the likely existence of compensatory activities of the three pocket proteins in mediating p16-induced senescence. However, our finding that the introduction of HPV E7 - which inactivates all three pocket proteins, prevented p16-induced senescence, is consistent with the hypothesis that as a group, pRb, p107 and p130 constitute essential mediators of p16-induced senescence. To examine the relative contributions of the pocket proteins to cellular senescence initiated by endogenous p16, we used cultured primary human mammary epithelial cells (HMEC) from non-malignant breast tissues. These cells spontaneously induce endogenous p16 and undergo senescence. When we examined the relative contributions of pocket proteins to senescence associated with increased endogenous p16 levels in HMEC, we found that suppressing pRb alone abrogated p16-induced senescence, whereas suppressing p107 or p130 did not. Remarkably, HMEC in which pRb alone was knocked down showed reduced p16 expression and longer proliferative life spans than controls or HMEC in which p107 or p130 were knocked down. These results suggest that pRb, but not p107 or p130, is required for full induction of endogenous p16 in breast cells, and provide a possible explanation for the special tumor-suppressor role of pRb.

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 3210.

BORIS (CTCFL) is not expressed in most human breast cell lines and high grade breast carcinomas

BORIS (CTCFL) is the only known paralog of the versatile regulatory protein CTCF, a multifunctional DNA binding protein that mediates distinct gene regulatory functions involved in cell growth, differentiation, and apoptosis. Unlike CTCF, the expression of BORIS is normally restricted to specific cells in testes (the only cells where CTCF is not expressed), where it may play a role in reprogramming the methylation pattern of male germ line DNA. Frequent amplification of the 20q13.2 region, which contains the BORIS gene, and expression of BORIS transcripts in diverse human tumors and cell lines have led to the hypothesis that aberrant expression of BORIS may play a role in tumorigenesis by interfering with CTCF functions. However, recent studies using more quantitative methods indicate low frequency of BORIS expression in melanoma, ovarian, prostate, and bladder carcinomas. To investigate the relationship between chromosome 20q13 amplification and BORIS mRNA levels within breast cancer cell lines and tissues, we developed a quantitative RT-PCR assay to measure the levels of BORIS mRNA. Endpoint RT-PCR assays were also used to investigate the possible expression of alternatively spliced variants. Using multiple primer sets and controls, we found that neither mature BORIS transcripts nor spliced variants are commonly expressed at detectable levels in malignant breast cells or tissues, although endogenous BORIS transcripts can be induced in MCF-7 cells following 5-aza-2′-deoxycytidine treatment. In conclusion, in most breast cancer cells, endogenous BORIS is unlikely to be expressed at sufficient levels to interfere with CTCF functions. Thus it is improbable that aberrant BORIS expression plays a role in most human breast cancers.

Multidimensional profiling of cell surface proteins and nuclear markers

Cell membrane proteins play an important role in tissue architecture and cell-cell communication. We hypothesize that segmentation and multidimensional characterization of the distribution of cell membrane proteins, on a cell-by-cell basis, enable improved classification of treatment groups and identify important characteristics that can otherwise be hidden. We have developed a series of computational steps to 1) delineate cell membrane protein signals and associate them with a specific nucleus; 2) compute a coupled representation of the multiplexed DNA content with membrane proteins; 3) rank computed features associated with such a multidimensional representation; 4) visualize selected features for comparative evaluation through heatmaps; and 5) discriminate between treatment groups in an optimal fashion. The novelty of our method is in the segmentation of the membrane signal and the multidimensional representation of phenotypic signature on a cell-by-cell basis. To test the utility of this method, the proposed computational steps were applied to images of cells that have been irradiated with different radiation qualities in the presence and absence of other small molecules. These samples are labeled for their DNA content and E-cadherin membrane proteins. We demonstrate that multidimensional representations of cell-by-cell phenotypes improve predictive and visualization capabilities among different treatment groups, and identify hidden variables.

Abstract B91: RNAi knockdown of Mek2, but not Mek1, increases proliferation of malignant and non-malignant human breast cells

We are using shRNAs against Mek1 and/or Mek2 to determine the individual contributions of Mek1 and Mek2 to proliferation, migration and Mek inhibitor sensitivity in malignant and non-malignant human breast epithelial cell lines. We found in both malignant (MCF7 and MDAMB231) and non-malignant immortalized (184vTERT and MCF10A) breast cell lines that, despite significant shRNA-mediated reductions (>95%) in the expression of Mek1, Mek2, or both kinases, the cells maintained long-term growth. In fact, the shMek2 lines grew significantly faster and were more motile than controls, suggesting that in some breast cancers, drugs that specifically target Mek1 might be more effective than those which target both Mek1 and 2.

In MCF7 cells, levels of phosphorylated (P)-Erk1 and 2 were significantly decreased or undetectable in the presence of Mek shRNAs, indicating that Erk phosphorylation may be dispensable for cell proliferation in these cells. In contrast, in 184vTERT and MCF10A, phosphorylated Erk levels remained constant despite the knockdown of both Mek1 and Mek2. This latter result indicated that, in some breast cells, another kinase is likely to phosphorylate Erk1/2 in the absence of Mek1 and Mek2. The Mek5 kinase is genetically most similar to Mek1 and Mek2, making it an attractive candidate for further investigation. Mek5 is expressed in 184vTERT, and its total protein levels are not altered when Mek1 and 2 are knocked down. A preliminary experiment indicates that siRNA induced knockdown of Mek5 expression leads to decreased ERK1/2 phosphorylation in 184v-TERT cells independently of Mek1/2 expression. We are currently investigating whether this occurs in other breast cell lines.

Collectively, our data indicate that in at least some breast cells, both Mek1 and Mek2 are dispensable for proliferation and survival, and that parallel pathways are sufficient for relaying signals from cell surface receptors, independently of Mek1/2 and/or P-Erk1/2. This data may explain why Mek inhibitors, which target both Mek1 and Mek2, have not done well clinically.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B91.

Telomerase activation by c-Myc in human mammary epithelial cells requires additional genomic changes

A central question in breast cancer biology is how cancer cells acquire telomerase activity required for unlimited proliferation. According to one model, proliferation of telomerase(-) pre-malignant cells leads to telomere dysfunction and increased genomic instability. Such instability leads in rare cases to reactivation of telomerase and immortalization. The mechanism of telomerase reactivation remains unknown. We have studied immortalization of cultured human mammary epithelial cells by c-Myc, a positive transcriptional regulator of the hTERT gene encoding the catalytic subunit of telomerase. Retrovirally introduced c-Myc cDNA resulted in immortalization of human mammary epithelial cells in which the cyclin dependent kinase inhibitor, p16(INK4A), was inactivated by an shRNA-encoding retrovirus. However, while c-Myc introduction immediately resulted in increased activity of transiently transfected hTERT promoter reporter constructs, endogenous hTERT mRNA levels did not change until about 60 population doublings after c-Myc introduction. Increased endogenous hTERT transcripts and stabilization of telomeric DNA in cells expressing exogenous c-Myc coincided with telomere dysfunction-associated senescence in control cultures. Genome copy number analyses of immortalized cells indicated amplifications of some or all of chromosome 5, where hTERT genes are located. hTERT gene copy number, however, was not increased in one case. The results are consistent with the hypothesis that changes in chromosome 5, while not necessarily increasing hTERT gene copy number, resulted in removal of repressive chromatin structures around hTERT loci, allowing induction of hTERT transcription. These in vitro results model one possible sequence of events leading to immortalization of breast epithelial cells during cancer progression.

Aging and cancer cell biology, 2009

Cancer is an age-related disease in organisms with renewable tissues. A malignant tumor arises in part from genomic damage, which can also drive age-related degeneration. However, cancer differs from many age-related degenerative diseases in that it entails gain-of-function changes that confer new (albeit aberrant) properties on cells, resulting in vigorous cell proliferation and survival. Nonetheless, interventions that delay age-related degeneration - for example, caloric restriction or dampened insulin/IGF-1 signaling - often also delay cancer. How then is the development of cancer linked to aging? The answer to this question is complex, as suggested by recent findings. This Hot Topic review discusses some of these findings, including how genomic damage might alter cellular properties without conferring mutations, and how some genes that regulate lifespan in organisms that lack renewable tissues might affect the development of cancer in mammals.

Interaction of E-cadherin and PTEN regulates morphogenesis and growth arrest in human mammary epithelial cells

Phosphatase and tensin homologue deleted on chromosome 10 (PTEN) is a dual-function phosphatase with tumor suppressor function compromised in a wide spectrum of cancers. Because tissue polarity and architecture are crucial modulators of normal and malignant behavior, we postulated that PTEN may play a role in maintenance of tissue integrity. We used two nonmalignant human mammary epithelial cell lines that form polarized, growth-arrested structures (acini) when cultured in three-dimensional laminin-rich extracellular matrix gels (lrECM). As acini begin to form, PTEN accumulates both in the cytoplasm and at cell-cell contacts where it colocalizes with the E-cadherin/beta-catenin complex. Reduction of PTEN levels by shRNA in lrECM prevents formation of organized breast acini and disrupts growth arrest. Importantly, disruption of acinar polarity and cell-cell contact by E-cadherin function-blocking antibodies reduces endogenous PTEN protein levels and inhibits its accumulation at cell-cell contacts. Conversely, in Skbr-3 breast cancer cells lacking endogenous E-cadherin expression, exogenous introduction of E-cadherin gene causes induction of PTEN expression and its accumulation at sites of cell interactions. These studies provide evidence that E-cadherin regulates both the PTEN protein levels and its recruitment to cell-cell junctions in three-dimensional lrECM, indicating a dynamic reciprocity between architectural integrity and the levels and localization of PTEN. This interaction thus seems to be a critical integrator of proliferative and morphogenetic signaling in breast epithelial cells.

Soothing the watchman: telomerase reduces the p53-dependent cellular stress response

In addition to conferring an indefinite replicative life span, telomerase renders p16(-) human mammary epithelial cells (HMEC) resistant to growth arrest by TGFbeta or by loss of EGF or insulin signaling. In contrast to earlier reports, we recently found that growth factor signaling was not directly affected by telomerase expression. Rather, short dysfunctional or near-dysfunctional telomeres in proliferating telomerase(-) HMEC sensitized the cells to p53-dependent signals for growth arrest. We showed that during serial passage and before any signs of replicative senescence, HMEC lacking telomerase experience enhanced p53 stability and DNA damage signaling, as determined by increased phosphorylation on p53-Ser15 and Chk2-Thr68, and formation of 53BP1/phosphorylated histone H2AX foci at chromosome ends. This heightened activity of the p53 pathway enhanced the efficiency with which cells arrested growth in response to TGFbeta or to EGF or insulin withdrawal, and was abolished by ectopic expression of hTERT, the catalytic subunit of telomerase. Telomerase elongated short telomeres, thereby reducing the basal level of activated p53 and raising cellular tolerance for other p53-dependent signals, including those emanating from non-genotoxic sources. These findings explain a number of observed effects of telomerase expression on cell growth and survival without postulating additional functions for telomerase.

Amplification of zinc finger gene 217 (ZNF217) and cancer: when good fingers go bad

Chromosome 20q13 is highly amplified in human cancers, including 20-30% of early stage human breast cancers. The amplification correlates with poor prognosis. Over-expression of the zinc-finger protein 217 (ZNF217), a candidate oncogene on 20q13.2, in cultured human mammary and ovarian epithelial cells can lead to their immortalization, indicating that selection for ZNF217 expression may drive 20q13 amplification during critical early stages of cancer progression. ZNF217 can also attenuate apoptotic signals resulting from exposure to doxorubicin, suggesting that ZNF217 expression may also be involved in resistance to chemotherapy. Recent findings indicate that ZNF217 binds specific DNA sequences, recruits the co-repressor C-terminal binding protein (CtBP), and represses the transcription of a variety of genes. Inappropriate expression of ZNF217 may lead to aberrant down-regulation of genes involved in limiting the proliferation, survival, and/or invasiveness of cancer cells. Better understanding of ZNF217 and its associated pathways may provide new targets for therapeutic intervention in human cancers.

Identification of genes directly regulated by the oncogene ZNF217 using chromatin immunoprecipitation (ChIP)-chip assays

It has been proposed that ZNF217, which is amplified at 20q13 in various tumors, plays a key role during neoplastic transformation. ZNF217 has been purified in complexes that contain repressor proteins such as CtBP2, suggesting that it acts as a transcriptional repressor. However, the function of ZNF217 has not been well characterized due to a lack of known target genes. Using a global chromatin immunoprecipitation (ChIP)-chip approach, we identified thousands of ZNF217 binding sites in three tumor cell lines (MCF7, SW480, and Ntera2). Further analysis of ZNF217 in Ntera2 cells showed that many promoters are bound by ZNF217 and CtBP2 and that a subset of these promoters are activated upon removal of ZNF217. Thus, our in vivo studies corroborate the in vitro biochemical analyses of ZNF217-containing complexes and support the hypothesis that ZNF217 functions as a transcriptional repressor. Gene ontology analysis showed that ZNF217 targets in Ntera2 cells are involved in organ development, suggesting that one function of ZNF217 may be to repress differentiation. Accordingly we show that differentiation of Ntera2 cells with retinoic acid led to down-regulation of ZNF217. Our identification of thousands of ZNF217 target genes will enable further studies of the consequences of aberrant expression of ZNF217 during neoplastic transformation.

Oncogene-induced senescence pathways weave an intricate tapestry

The induction of cellular senescence by activated oncogenes acts as a barrier to cell transformation. Now, identify a key component of a senescence pathway that prevents tumorigenesis in a mouse model of skin cancer. They show that the p38-regulated/activated protein kinase (PRAK) induces senescence downstream of oncogenic Ras by directly phosphorylating and activating the tumor-suppressor protein p53.

p53-dependent integration of telomere and growth factor deprivation signals

Ectopically expressed hTERT enables p16(INK4A)(-) human mammary epithelial cells to proliferate in the absence of growth factors, a finding that has led to the hypothesis that hTERT has growth regulatory properties independent of its role in telomere maintenance. We now show that telomerase can alter the growth properties of cells indirectly through its role in telomere maintenance, without altering growth stimulatory pathways. We find that telomere dysfunction, indicated by 53BP1/phosphorylated histone H2AX foci at chromosome ends, is present in robustly proliferating human mammary epithelial cells long before senescence. These foci correlate with increased levels of active p53. Ectopic expression of hTERT reduces the number of foci and the level of active p53, thereby decreasing sensitivity to growth factor depletion, which independently activates p53. The continuous presence of hTERT is not necessary for this effect, indicating that telomere maintenance, rather than the presence of the enzyme itself, is responsible for the increased ability to proliferate in the absence of growth factors. Our findings provide a previously unrecognized mechanistic explanation for the observation that ectopically expressed hTERT conveys growth advantages to cells, without having to postulate nontelomeric functions for the enzyme.

Gene expression signature in organized and growth-arrested mammary acini predicts good outcome in breast cancer

Nonmalignant human mammary epithelial cells (HMEC) seeded in laminin-rich extracellular matrix (lrECM) form polarized acini and, in doing so, transit from a disorganized proliferating state to an organized growth-arrested state. We hypothesized that the gene expression pattern of organized and growth-arrested HMECs would share similarities with breast tumors with good prognoses. Using Affymetrix HG-U133A microarrays, we analyzed the expression of 22,283 gene transcripts in 184 (finite life span) and HMT3522 S1 (immortal nonmalignant) HMECs on successive days after seeding in a lrECM assay. Both HMECs underwent growth arrest in G0-G1 and differentiated into polarized acini between days 5 and 7. We identified gene expression changes with the same temporal pattern in both lines and examined the expression of these genes in a previously published panel of microarray data for 295 breast cancer samples. We show that genes that are significantly lower in the organized, growth-arrested HMEC than in their proliferating counterparts can be used to classify breast cancer patients into poor and good prognosis groups with high accuracy. This study represents a novel unsupervised approach to identifying breast cancer markers that may be of use clinically.

Specific recognition of ZNF217 and other zinc finger proteins at a surface groove of C-terminal binding proteins

Numerous transcription factors recruit C-terminal binding protein (CtBP) corepressors. We show that the large zinc finger protein ZNF217 contacts CtBP. ZNF217 is encoded by an oncogene frequently amplified in tumors. ZNF217 contains a typical Pro-X-Asp-Leu-Ser (PXDLS) motif that binds in CtBP's PXDLS-binding cleft. However, ZNF217 also contains a second motif, Arg-Arg-Thr (RRT), that binds a separate surface on CtBP. The crystal structure of CtBP bound to an RRTGAPPAL peptide shows that it contacts a surface crevice distinct from the PXDLS binding cleft. Interestingly, both PXDLS and RRT motifs are also found in other zinc finger proteins, such as RIZ. Finally, we show that ZNF217 represses several promoters, including one from a known CtBP target gene, and mutations preventing ZNF217's contact with CtBP reduce repression. These results identify a new CtBP interaction motif and establish ZNF217 as a transcriptional repressor protein that functions, at least in part, by associating with CtBP.

ZNF217 suppresses cell death associated with chemotherapy and telomere dysfunction

Chromosome 20q13.2 is amplified in 20-30% of early-stage breast tumors and is associated with poor prognosis. Detailed mapping of the amplified region using molecular cytogenetics, positional cloning and genomic sequencing culminated in a detailed molecular description of the candidate oncogene ZNF217. ZNF217 proteins resemble Kruppel-like transcription factors, localize predominately to the nucleus and associate with proteins involved in transcriptional repression. The findings that ZNF217 can immortalize human mammary epithelial cells and that its amplification is associated with poor prognosis suggest that it may play roles in both early- and late-stage breast cancer. We present evidence that ZNF217 can attenuate apoptotic signals resulting from telomere dysfunction as well as from doxorubicin-induced DNA damage and that silencing ZNF217 with siRNA restores sensitivity to doxorubicin. Moreover, elevated ZNF217 leads to increased phosphorylation of Akt, whereas inhibition of the phosphatidylinositol 3 kinase pathway and Akt phosphorylation decreases ZNF217 protein levels and increases sensitivity to doxorubicin. These results suggest that ZNF217 may promote neoplastic transformation by increasing cell survival during telomeric crisis and may promote later stages of malignancy by increasing cell survival during chemotherapy.

Cancer and aging: the importance of telomeres in genome maintenance

Telomeres are the specialized DNA-protein structures that cap the ends of linear chromosomes, thereby protecting them from degradation and fusion by cellular DNA repair processes. In vertebrate cells, telomeres consist of several kilobase pairs of DNA having the sequence TTAGGG, a few hundred base pairs of single-stranded DNA at the 3' end of the telomeric DNA tract, and a host of proteins that organize the telomeric double and single-stranded DNA into a protective structure. Functional telomeres are essential for maintaining the integrity and stability of genomes. When combined with loss of cell cycle checkpoint controls, telomere dysfunction can lead to genomic instability, a common cause and hallmark of cancer. Consequently, normal mammalian cells respond to dysfunctional telomeres by undergoing apoptosis (programmed cell death) or cellular senescence (permanent cell cycle arrest), two cellular tumor suppressor mechanisms. These tumor suppressor mechanisms are potent suppressors of cancer, but recent evidence suggests that they can antagonistically also contribute to aging phenotypes. Here, we review what is known about the structure and function of telomeres in mammalian cells, particularly human cells, and how telomere dysfunction may arise and contribute to cancer and aging phenotypes.

Accumulation and altered localization of telomere-associated protein TRF2 in immortally transformed and tumor-derived human breast cells

We have used cultured human mammary epithelial cells (HMEC) and breast tumor-derived lines to gain information on defects that occur during breast cancer progression. HMEC immortalized by a variety of agents (the chemical carcinogen benzo(a)pyrene, oncogenes c-myc and ZNF217, and/or dominant negative p53 genetic suppressor element GSE22) displayed marked upregulation (10-15 fold) of the telomere-binding protein, TRF2. Upregulation of TRF2 protein was apparently due to differences in post-transcriptional regulation, as mRNA levels remained comparable in finite lifespan and immortal HMEC. TRF2 protein was not upregulated by the oncogenic agents alone in the absence of immortalization, nor by expression of exogenously introduced hTERT genes. We found TRF2 levels to be at least twofold higher than in control cells in 11/15 breast tumor cell lines, suggesting that elevated TRF2 levels are a frequent occurrence during the transformation of breast tumor cells in vivo. The dispersed distribution of TRF2 throughout the nuclei in some immortalized and tumor-derived cells indicated that not all the TRF2 was associated with telomeres in these cells. The process responsible for accumulation of TRF2 in immortalized HMEC and breast tumor-derived cell lines may promote tumorigenesis by contributing to the cells' ability to maintain an indefinite lifespan.

In situ analyses of genome instability in breast cancer

Transition through telomere crisis is thought to be a crucial event in the development of most breast carcinomas. Our goal in this study was to determine where this occurs in the context of histologically defined breast cancer progression. To this end, we assessed genome instability (using fluorescence in situ hybridization) and other features associated with telomere crisis in normal ductal epithelium, usual ductal hyperplasia, ductal carcinoma in situ and invasive cancer. We modeled this process in vitro by measuring these same features in human mammary epithelial cell cultures during ZNF217-mediated transition through telomere crisis and immortalization. Taken together, the data suggest that transition through telomere crisis and immortalization in breast cancer occurs during progression from usual ductal hyperplasia to ductal carcinoma in situ.

Reversal of human cellular senescence: roles of the p53 and p16 pathways

Telomere erosion and subsequent dysfunction limits the proliferation of normal human cells by a process termed replicative senescence. Replicative senescence is thought to suppress tumorigenesis by establishing an essentially irreversible growth arrest that requires activities of the p53 and pRB tumor suppressor proteins. We show that, depending on expression of the pRB regulator p16, replicative senescence is not necessarily irreversible. We used lentiviruses to express specific viral and cellular proteins in senescent human fibroblasts and mammary epithelial cells. Expression of telomerase did not reverse the senescence arrest. However, cells with low levels of p16 at senescence resumed robust growth upon p53 inactivation, and limited growth upon expression of oncogenic RAS. In contrast, cells with high levels of p16 at senescence failed to proliferate upon p53 inactivation or RAS expression, although they re-entered the cell cycle without growth after pRB inactivation. Our results indicate that the senescence response to telomere dysfunction is reversible and is maintained primarily by p53. However, p16 provides a dominant second barrier to the unlimited growth of human cells.

Loss of p53 function accelerates acquisition of telomerase activity in indefinite lifespan human mammary epithelial cell lines

We describe novel effects of p53 loss on immortal transformation, based upon comparison of immortally transformed human mammary epithelial cell (HMEC) lines lacking functional p53 with closely related p53(+) lines. Our previous studies of p53(+) immortal HMEC lines indicated that overcoming the stringent replicative senescence step associated with critically short telomeres (agonescence), produced indefinite lifespan lines that maintained growth without immediately expressing telomerase activity. These telomerase(-) 'conditionally immortal' HMEC underwent an additional step, termed conversion, to become fully immortal telomerase(+) lines with uniform good growth. The very gradual conversion process was associated with slow heterogeneous growth and high expression of the cyclin-dependent kinase inhibitor p57(Kip2). We now show that p53 suppresses telomerase activity and is necessary for the p57 expression in early passage p53(+) conditionally immortal HMEC lines, and that p53(-/-) lines exhibit telomerase reactivation and attain full immortality much more rapidly. A p53-inhibiting genetic suppressor element introduced into early passages of a conditionally immortal telomerase(-) p53(+) HMEC line led to rapid induction of hTERT mRNA, expression of telomerase activity, loss of p57 expression, and quick attainment of uniform good growth. These studies indicate that derangements in p53 function may impact malignant progression through direct effects on the conversion process, a potentially rate-limiting step in HMEC acquisition of uniform unlimited growth potential. These studies also provide evidence that the function of p53 in suppression of telomerase activity is separable from its cell cycle checkpoint function.

Molecular changes accompanying senescence and immortalization of cultured human mammary epithelial cells

Limits on the proliferative potential of cultured normal human cells may be consequences of pathways that exist to suppress tumorigenicity. Human mammary epithelial cells (HMEC) employ several mechanisms to prevent unlimited growth. One mechanism may be activated by stress, and is associated with upregulated expression of p16(INK4a). In serum-free medium, some HMEC arise spontaneously which do not express p16. These "post-selection" HMEC are capable of long-term proliferation, but ultimately cease growth when their telomeres become very short. As they approach a growth plateau, termed agonescence, post-selection HMEC populations accumulate chromosome abnormalities. In contrast to the crisis exhibited by cells lacking functional p53, agonescent cells can be maintained as viable cultures. Although transduction of hTERT, the catalytic subunit of telomerase, into post-selection cells can, by itself, efficiently produce immortality and avoid agonescence, the errors that produce telomerase reactivation during carcinogenesis are not known. The block to endogenous telomerase reactivation in HMEC is extremely stringent. However, if one predisposing error is present, the probability greatly increases that additional error(s) required for immortalization may be generated by genomic instability encountered during agonescence. In p53(+) HMEC immortalized after chemical carcinogen exposure, the events involved in overcoming agonescence can be temporally separated from activation of telomerase. We have used the term "conversion" to describe the gradual process that leads to telomerase activation, telomere length stabilization, decreased p57 (KIP2) expression, and increased ability to grow uniformly well in the presence or absence of TGF beta. In the presence of active p53, conversion may represent a rate-limiting step in immortal transformation.

Raf-1-induced growth arrest in human mammary epithelial cells is p16-independent and is overcome in immortal cells during conversion

Using an estrogen-inducible retroviral system, we demonstrate that oncogenic Raf-1 induces growth arrest and morphological changes in finite lifespan human mammary epithelial cells (HMEC). This arrest does not rely on expression of the cyclin-dependent kinase inhibitor (CKI) p16(INK4a), nor on changes in expression of the CKIs p21(Cip1), p14(ARF), p27(Kip1) or p57(Kip2). The Raf-induced arrest is independent of viral oncogene mediated inactivation of p53 and pRB, or c-myc overexpression. Flow cytometric analysis demonstrates that cells arrest in both G1 and G2. The Raf-induced arrest is mitigated or eliminated in some immortally transformed HMEC. Immortal HMEC that have both overcome replicative senescence and undergone the recently described conversion process maintain growth in the presence of transduced oncogenic Raf-1; they also gain EGF-independent growth and a low frequency of anchorage-independent growth. However, HMEC that have overcome replicative senescence but have not undergone conversion and HMEC immortalized by transduction with the catalytic subunit of telomerase, hTERT, remain severely growth arrested. These results indicate that the molecular mechanisms responsible for the Raf-1-induced growth arrest may vary among different finite lifespan cell types, and that in HMEC, this mechanism is altered during the conversion process, rather than as a direct consequence of overcoming senescence or expressing hTERT.

Crystal structure of human calmodulin-like protein: insights into its functional role

A calmodulin (CaM)-like protein (hCLP) is expressed in human mammary epithelial cells but appears to be limited to certain epithelial cells such as those found in skin, prostate, breast and cervical tissues. A decrease in the expression of this protein is associated with the occurrence of tumors in breast epithelium. The structure of hCLP determined to 1.5 A resolution by X-ray crystallography shows a distinct 30 degrees displacement along the interconnecting central helix, when compared to the highly conserved structure of vertebrate CaM, resulting in a difference in the relative orientation of its two globular domains. Additionally, the electric surface potential landscape at the target protein binding regions on the two globular domains of hCLP is significantly different from those of CaM, indicating that the respective ranges of hCLP and hCaM target proteins do not fully overlap. Observations that hCLP can competitively inhibit CaM activation of target proteins also imply a role for hCLP in which it may also serve as a modulator of CaM activity in the epithelial cells where hCLP is expressed.

TANK2, a new TRF1-associated poly(ADP-ribose) polymerase, causes rapid induction of cell death upon overexpression

Tankyrase (TANK1) is a human telomere-associated poly(ADP-ribose) polymerase (PARP) that binds the telomere-binding protein TRF1 and increases telomere length when overexpressed. Here we report characterization of a second human tankyrase, tankyrase 2 (TANK2), which can also interact with TRF1 but has properties distinct from those of TANK1. TANK2 is encoded by a 66-kilobase pair gene (TNKS2) containing 28 exons, which express a 6.7-kilobase pair mRNA and a 1166-amino acid protein. The protein shares 85% amino acid identity with TANK1 in the ankyrin repeat, sterile alpha-motif, and PARP catalytic domains but has a unique N-terminal domain, which is conserved in the murine TNKS2 gene. TANK2 interacted with TRF1 in yeast and in vitro and localized predominantly to a perinuclear region, similar to the properties of TANK1. In contrast to TANK1, however, TANK2 caused rapid cell death when highly overexpressed. TANK2-induced death featured loss of mitochondrial membrane potential, but not PARP1 cleavage, suggesting that TANK2 kills cells by necrosis. The cell death was prevented by the PARP inhibitor 3-aminobenzamide. In vivo, TANK2 may differ from TANK1 in its intrinsic or regulated PARP activity or its substrate specificity.