INHBA(+) cancer-associated fibroblasts generate an immunosuppressive tumor microenvironment in ovarian cancer

Effective targeting of cancer-associated fibroblasts (CAFs) is hindered by the lack of specific biomarkers and a poor understanding of the mechanisms by which different populations of CAFs contribute to cancer progression. While the role of TGFβ in CAFs is well-studied, less attention has been focused on a structurally and functionally similar protein, Activin A (encoded by INHBA). Here, we identified INHBA(+) CAFs as key players in tumor promotion and immunosuppression. Spatiotemporal analyses of patient-matched primary, metastatic, and recurrent ovarian carcinomas revealed that aggressive metastatic tumors enriched in INHBA(+) CAFs were also enriched in regulatory T cells (Tregs). In ovarian cancer mouse models, intraperitoneal injection of the Activin A neutralizing antibody attenuated tumor progression and infiltration with pro-tumorigenic subsets of myofibroblasts and macrophages. Downregulation of INHBA in human ovarian CAFs inhibited pro-tumorigenic CAF functions. Co-culture of human ovarian CAFs and T cells revealed the dependence of Treg differentiation on direct contact with INHBA(+) CAFs. Mechanistically, INHBA/recombinant Activin A in CAFs induced the autocrine expression of PD-L1 through SMAD2-dependent signaling, which promoted Treg differentiation. Collectively, our study identified an INHBA(+) subset of immunomodulatory pro-tumoral CAFs as a potential therapeutic target in advanced ovarian cancers which typically show a poor response to immunotherapy.

Are Epithelial Ovarian Cancers of the Mesenchymal Subtype Actually Intraperitoneal Metastases to the Ovary?

Primary ovarian high-grade serous carcinoma (HGSC) has been classified into 4 molecular subtypes: Immunoreactive, Proliferative, Differentiated, and Mesenchymal (Mes), of which the Mes subtype (Mes-HGSC) is associated with the worst clinical outcomes. We propose that Mes-HGSC comprise clusters of cancer and associated stromal cells that detached from tumors in the upper abdomen/omentum and disseminated in the peritoneal cavity, including to the ovary. Using comparative analyses of multiple transcriptomic data sets, we provide the following evidence that the phenotype of Mes-HGSC matches the phenotype of tumors in the upper abdomen/omentum: (1) irrespective of the primary ovarian HGSC molecular subtype, matched upper abdominal/omental metastases were typically of the Mes subtype, (2) the Mes subtype was present at the ovarian site only in patients with concurrent upper abdominal/omental metastases and not in those with HGSC confined to the ovary, and (3) ovarian Mes-HGSC had an expression profile characteristic of stromal cells in the upper abdominal/omental metastases. We suggest that ovarian Mes-HGSC signifies advanced intraperitoneal tumor dissemination to the ovary rather than a subtype of primary ovarian HGSC. This is consistent with the presence of upper abdominal/omental disease, suboptimal debulking, and worst survival previously reported in patients with ovarian Mes-HGSC compared to other molecular subtypes.

Tumors defective in homologous recombination rely on oxidative metabolism: relevance to treatments with PARP inhibitors

Mitochondrial metabolism and the generation of reactive oxygen species (ROS) contribute to the acquisition of DNA mutations and genomic instability in cancer. How genomic instability influences the metabolic capacity of cancer cells is nevertheless poorly understood. Here, we show that homologous recombination‐defective (HRD) cancers rely on oxidative metabolism to supply NAD⁺ and ATP for poly(ADP‐ribose) polymerase (PARP)‐dependent DNA repair mechanisms. Studies in breast and ovarian cancer HRD models depict a metabolic shift that includes enhanced expression of the oxidative phosphorylation (OXPHOS) pathway and its key components and a decline in the glycolytic Warburg phenotype. Hence, HRD cells are more sensitive to metformin and NAD⁺ concentration changes. On the other hand, shifting from an OXPHOS to a highly glycolytic metabolism interferes with the sensitivity to PARP inhibitors (PARPi) in these HRD cells. This feature is associated with a weak response to PARP inhibition in patient‐derived xenografts, emerging as a new mechanism to determine PARPi sensitivity. This study shows a mechanistic link between two major cancer hallmarks, which in turn suggests novel possibilities for specifically treating HRD cancers with OXPHOS inhibitors.

Glucose deprivation elicits phenotypic plasticity via ZEB1-mediated expression of NNMT

Glucose is considered the primary energy source for all cells, and some cancers are addicted to glucose. Here, we investigated the functional consequences of chronic glucose deprivation in serous ovarian cancer cells. We found that cells resistant to glucose starvation (glucose-restricted cells) demonstrated increased metabolic plasticity that was dependent on NNMT (Nicotinamide N-methyltransferase) expression. We further show that ZEB1 induced NNMT, rendered cells resistant to glucose deprivation and recapitulated metabolic adaptations and mesenchymal gene expression observed in glucose-restricted cells. NNMT depletion reversed metabolic plasticity in glucose-restricted cells and prevented de novo formation of glucose-restricted colonies. In addition to its role in glucose independence, we found that NNMT was required for other ZEB1-induced phenotypes, such as increased migration. NNMT protein levels were also elevated in metastatic and recurrent tumors compared to matched primary carcinomas, while normal ovary and fallopian tube tissue had no detectable NNMT expression. Our studies define a novel ZEB1/NNMT signaling axis, which elicits mesenchymal gene expression, as well as phenotypic and metabolic plasticity in ovarian cancer cells upon chronic glucose starvation. Understanding the causes of cancer cell plasticity is crucial for the development of therapeutic strategies to counter intratumoral heterogeneity, acquired drug resistance and recurrence in high-grade serous ovarian cancer (HGSC).

Abstract TMEM-026: ZEB1–MEDIATED NNMT EXPRESSION ELICITS PHENOTYPIC AND METABOLIC PLASTICITY OF OVARIAN CANCER CELLS UNDER NUTRITIONAL STRESS

Glucose is considered the primary energy source for all cells, and some cancers are addicted to glucose. However, poorly vascularized regions of advanced human cancers, such as high grade serous ovarian cancers (HGSC), may have limited glucose access and suffer from nutritional stress. Here, we investigated the functional consequences of chronic glucose deprivation in serous ovarian cancer cell lines (OVCAR3, OVCAR4 and OAW28).

Cells selected for resistance to glucose deprivation (glucose-independent sublines) demonstrated increased metabolic plasticity in utilization of additional energy sources compared to glucose-dependent precursor cells. Furthermore, some cells acquired mesenchymal gene expression and functional characteristics of EMT, such as increased migration efficiency; however classic EMT was not a prerequisite for glucose independence as majority of sublines retained their epithelial-like morphology. Instead, we identified the metabolic enzyme Nicotinamide N-methyltransferase (NNMT) to be commonly upregulated in all glucose-independent sublines. NNMT catalyzes methylation reactions that may alter histones and DNA methylation status. Also, NNMT expression is enriched in the mesenchymal subtype of HGSC and correlates with an invasive phenotype in vitro. We showed that NNMT overexpression in glucose-independent cells is mediated by genetic changes (genomic gain of NNMT), as well as transcriptional changes, such as induction of ZEB1. Ectopic expression of ZEB1 in ovarian cancer cell lines strongly induced NNMT, rendered cells more resistant to glucose deprivation and recapitulated many of the metabolic adaptations and mesenchymal gene expression observed in glucose-independent sublines. We further showed that ZEB1-driven glucose independence was mediated by NNMT because depletion of NNMT via shRNA or CRISPR/Cas9 reversed the metabolic plasticity of these cells and significantly impaired proliferation and de novo formation of glucose-independent colonies in the absence of glucose. In addition to its role in glucose independence, we found that NNMT was required for other ZEB1-induced phenotypes, such as increased migration.

Our analysis also showed that increased NNMT expression correlated with worse overall and progression-free survival in patients. In line with these results, our tissue microarray data demonstrated that NNMT protein levels were elevated in metastatic and recurrent tumors compared to matched primary carcinomas, while normal ovary and fallopian tube tissue had no detectable NNMT expression.

Our studies define a novel ZEB1/NNMT signaling axis, which elicits phenotypic and metabolic plasticity of ovarian cancer cells upon chronic glucose starvation. Understanding the causes of cancer cell plasticity is crucial for the development of therapeutic strategies to counter intratumoral heterogeneity, acquired drug resistance and recurrence in HGSC.

Citation Format: Justyna Kanska, Barbie Taylor-Harding, Paul-Joseph Aspuria, Beth Y. Karlan, Simon Gayther and W. Ruprecht Wiedemeyer. ZEB1–MEDIATED NNMT EXPRESSION ELICITS PHENOTYPIC AND METABOLIC PLASTICITY OF OVARIAN CANCER CELLS UNDER NUTRITIONAL STRESS [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr TMEM-026.

A COL11A1-correlated pan-cancer gene signature of activated fibroblasts for the prioritization of therapeutic targets

Although cancer-associated fibroblasts (CAFs) are viewed as a promising therapeutic target, the design of rational therapy has been hampered by two key obstacles. First, attempts to ablate CAFs have resulted in significant toxicity because currently used biomarkers cannot effectively distinguish activated CAFs from non-cancer associated fibroblasts and mesenchymal progenitor cells. Second, it is unclear whether CAFs in different organs have different molecular and functional properties that necessitate organ-specific therapeutic designs. Our analyses uncovered COL11A1 as a highly specific biomarker of activated CAFs. Using COL11A1 as a 'seed', we identified co-expressed genes in 13 types of primary carcinoma in The Cancer Genome Atlas. We demonstrated that a molecular signature of activated CAFs is conserved in epithelial cancers regardless of organ site and transforming events within cancer cells, suggesting that targeting fibroblast activation should be effective in multiple cancers. We prioritized several potential pan-cancer therapeutic targets that are likely to have high specificity for activated CAFs and minimal toxicity in normal tissues.

Screening cell mechanotype by parallel microfiltration

Cell mechanical phenotype or 'mechanotype' is emerging as a valuable label-free biomarker. For example, marked changes in the viscoelastic characteristics of cells occur during malignant transformation and cancer progression. Here we describe a simple and scalable technique to measure cell mechanotype: this parallel microfiltration assay enables multiple samples to be simultaneously measured by driving cell suspensions through porous membranes. To validate the method, we compare the filtration of untransformed and HRas(V12)-transformed murine ovary cells and find significantly increased deformability of the transformed cells. Inducing epithelial-to-mesenchymal transition (EMT) in human ovarian cancer cells by overexpression of key transcription factors (Snail, Slug, Zeb1) or by acquiring drug resistance produces a similar increase in deformability. Mechanistically, we show that EMT-mediated changes in epithelial (loss of E-Cadherin) and mesenchymal markers (vimentin induction) correlate with altered mechanotype. Our results demonstrate a method to screen cell mechanotype that has potential for broader clinical application.

Cyclin E1 and RTK/RAS signaling drive CDK inhibitor resistance via activation of E2F and ETS

High-grade serous ovarian cancers (HGSOC) are genomically complex, heterogeneous cancers with a high mortality rate, due to acquired chemoresistance and lack of targeted therapy options. Cyclin-dependent kinase inhibitors (CDKi) target the retinoblastoma (RB) signaling network, and have been successfully incorporated into treatment regimens for breast and other cancers. Here, we have compared mechanisms of response and resistance to three CDKi that target either CDK4/6 or CDK2 and abrogate E2F target gene expression. We identify CCNE1 gain and RB1 loss as mechanisms of resistance to CDK4/6 inhibition, whereas receptor tyrosine kinase (RTK) and RAS signaling is associated with CDK2 inhibitor resistance. Mechanistically, we show that ETS factors are mediators of RTK/RAS signaling that cooperate with E2F in cell cycle progression. Consequently, CDK2 inhibition sensitizes cyclin E1-driven but not RAS-driven ovarian cancer cells to platinum-based chemotherapy. In summary, this study outlines a rational approach for incorporating CDKi into treatment regimens for HGSOC.

VEGFR3 inhibition chemosensitizes ovarian cancer stemlike cells through down-regulation of BRCA1 and BRCA2

In ovarian cancer, loss of BRCA gene expression in tumors is associated with improved response to chemotherapy and increased survival. A means to pharmacologically downregulate BRCA gene expression could improve the outcomes of patients with BRCA wild-type tumors. We report that vascular endothelial growth factor receptor 3 (VEGFR3) inhibition in ovarian cancer cells is associated with decreased levels of both BRCA1 and BRCA2. Inhibition of VEGFR3 in ovarian tumor cells was associated with growth arrest. CD133(+) ovarian cancer stemlike cells were preferentially susceptible to VEGFR3-mediated growth inhibition. VEGFR3 inhibition-mediated down-regulation of BRCA gene expression reversed chemotherapy resistance and restored chemosensitivity in resistant cell lines in which a BRCA2 mutation had reverted to wild type. Finally, we demonstrate that tumor-associated macrophages are a primary source of VEGF-C in the tumor microenvironment. Our studies suggest that VEGFR3 inhibition may be a pharmacologic means to downregulate BRCA genes and improve the outcomes of patients with BRCA wild-type tumors.

Abstract 678: The Ets factor ETV5 regulates E2F target genes and mediates drug resistance

E2F transcription factors are downstream mediators of several oncogenic drivers, including CDK/cyclin complexes, MYC, and mitogen-activated kinase (MAPK) signaling. E2F transcribe genes involved in G1-S phase transition and couple cell cycle progression with DNA repair. Prominent E2F target genes include CDK1 (CDC2), cyclin E1 (CCNE1) and BRCA1. Cyclin E1 is part of a positive feedback loop: its transcription is induced by activator E2F. Cyclin E1 protein then binds to CDK2, and the complex inactivates the RB protein, a potent inhibitor of activator E2F. The CCNE1 gene is frequently amplified in high-grade serous ovarian cancer (HGSOC, 20%) and other cancers. In the TCGA dataset on HGSOC, E2F target genes, including BRCA1, are co-expressed with CCNE1, and CCNE1-amplified HGSOC overexpress BRCA1. Moreover, a recent study suggests that CCNE1-amplified HGSOC are critically dependent on BRCA1. Loss of BRCA1 function results in synthetic lethality specifically in this subset. This finding provides an explanation for the mutually exclusive relationship between CCNE1 amplification and BRCA1 mutation.

Based on the functional link between cyclin E1 and BRCA1, we devised a targeting strategy for cyclin E1-dependent ovarian cancers using pharmacological CDK2 inhibitors. Mutations in BRCA1 and BRCA2 sensitize cancer cells to platinum-based chemotherapy. We hypothesized that CDK inhibitors (CDKi) would induce loss of BRCA function and chemosensitize cyclin E1-dependent HGSOC. Indeed, we found that the CDK1/2 inhibitor, Dinaciclib, sensitized cyclin E1-transformed p53-null murine ovarian surface epithelial (MOSE) cells to cisplatin, whereas no sensitization was observed in HRASV12-driven MOSE cells. Similarly, progression of human CCNE1-amplified OVCAR3 xenografts was blocked by combined treatment with Dinaciclib and cisplatin but not by either single agent. Mechanistically, Dinaciclib treatment resulted in transcriptional downregulation of both BRCA1 and cyclin E1. Comparison of CDKi-sensitive and CDKi-resistant ovarian cancer cell lines revealed that the Ets family transcription factor, ETV5, was highly expressed in resistant lines. ETV5 is a downstream mediator of MAPK signaling and known oncogene in prostate cancer.

Using inducible shRNA, we show that genetic depletion of ETV5 in RAS-driven ovarian cancer cells reduces tumorigenicity, accompanied by transcriptional downregulation of cyclin E1 and BRCA1. Thus, ETV5 regulates a subset of E2F target genes. These findings suggest that Ets family transcription factors can compensate for loss of E2F function induced by targeted agents, such as CDKi. Functional compensation on the level of receptor tyrosine kinases has been described as a mechanism of drug resistance. Our study demonstrates similar compensatory mechanisms among transcription factors that mediate oncogenic signaling.

Citation Format: Barbie Taylor-Harding, Hasmik Agadjanian, Paul-Joseph Aspuria, Dong-Joo Cheon, Takako Mizuno, Danielle Greenberg, Jenieke R. Allen, Sandra Orsulic, Christine Walsh, Beth Y. Karlan, W. Ruprecht Wiedemeyer. The Ets factor ETV5 regulates E2F target genes and mediates drug resistance. [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 678. doi:10.1158/1538-7445.AM2014-678

Abstract B48: Targeting chemo-resistance in CCNE1-amplified ovarian cancer

The putative oncogene CCNE1 (cyclin E1) is frequently amplified in human high-grade serous ovarian cancer. CCNE1-amplified cancers tend to be wildtype for BRCA1 and BRCA2 and are associated with shorter survival and resistance to platinum-based chemotherapy. Cyclin E1 is an activating cofactor for cyclin-dependent kinases (CDK), which stimulate cell cycle progression through phosphorylation of the retinoblastoma (RB) protein and subsequent induction of E2F transcriptional activity (cyclin E1-RB-E2F signaling). Since BRCA1 and BRCA2 are known target genes of E2F transcription factors, we hypothesize that cyclin E1 signaling actively contributes to high DNA repair capacity and chemo-resistance in a BRCA1/2-dependent manner. We predict CCNE1-amplified ovarian cancers to exhibit an “anti-BRCAness” phenotype. In two independent gene expression datasets, generated by TCGA and The Women's Cancer Program at Cedars-Sinai, we found that CCNE1-amplified ovarian cancers maintain higher levels of BRCA1 than CCNE1-wildtype tumors. Moreover, we have employed genetic and pharmacological targeting approaches in order to assess if inhibition of cyclin E1-dependent signaling can induce BRCAness and restore chemo-sensitivity. Here, we show that pharmacological CDK inhibitors (CDKi) effectively shut down E2F-mediated transcription, resulting in downregulation of BRCA1 and BRCA2 in CDKi-treated cells. Long-term exposure of ovarian cancer cell lines to CDKi selected for cells with reduced dependency on cyclin E1, and genomic profiling revealed de novo DNA copy number changes which compensate for loss of cyclin E1 function. Importantly, CDKi-resistant subclones retained lower levels of BRCA1 and were significantly more sensitive to cisplatin than parental cell lines. Collectively, our results suggest that cyclin E1 signaling is required for BRCA1 expression and chemo-resistance. Currently available CDKi such as Dinaciclib, which is in a phase 3 clinical trial, may be useful to specifically sensitize CCNE1-amplified ovarian cancers to cisplatin.

Citation Format: Barbie Taylor-Harding, Hasmik Agadjanian, Paul Joseph Aspuria, Takako Mizuno, Dong-Joo Cheon, Sandra Orsulic, Beth Karlan, Christine Walsh, Wolf Ruprecht Wiedemeyer. Targeting chemo-resistance in CCNE1-amplified ovarian cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: From Concept to Clinic; Sep 18-21, 2013; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2013;19(19 Suppl):Abstract nr B48.

Abstract 1749: Cell cycle requirements shape ovarian cancer progression

Acquired resistance to chemotherapeutic drugs and subsequent recurrence of drug-resistant cancer are serious challenges associated with epithelial ovarian cancer. Most high-grade tumors initially respond well to the combination of platinum and taxane-based agents but recur as platinum-resistant disease, which is incurable. In addition, a subset of primary tumors, including CCNE1-amplified tumors, are inherently less susceptible to standard chemotherapy. Studies by The Cancer Genome Atlas have revealed that CCNE1 (cyclin E) amplification is mutually exclusive with mutations in BRCA1 and BRCA2, which render ovarian cancer cells sensitive to DNA-damaging agents and PARP inhibitors, as a result of synthetic lethality. In order to develop a similar therapeutic concept for CCNE1-amplified cancers, we have studied responses and resistance mechanisms of human ovarian cancer cells to several pharmacological inhibitors of cyclin-dependent kinases (CDK). Functional and biochemical studies have established cyclin E as an activator of CDK, which phosphorylate the RB tumor suppressor. However, despite the functional importance of the RB pathway in cancer, CDK inhibitors (CDKi) have yet to make a significant impact on the management of solid tumors.

Here, we describe oncogenic pathways that are activated in CDKi-resistant cells and play critical roles in tumor progression and drug resistance. Targeting individual CDK or subsets of multiple CDK in vitro and in vivo, we found that cyclin E dependence was the main driving force that shaped tumor progression in different model systems of CCNE1-amplified and recurrent ovarian cancer. We identified robust and specific compensatory mechanisms that sustain cyclin E-associated activity in CDKi-resistant cells via activation of oncogenic signaling upstream of the cyclin E-RB-E2F axis. Specific targeting of cooperating oncogenes increased CDKi efficacy and prevented the outgrowth of CDKi-resistant cancer cells. Collectively, our results suggest that CDKi can be highly effective elements of combination therapy.

Citation Format: Barbie Taylor-Harding, Hasmik Agadjanian, Sandra Orsulic, Christine Walsh, Beth Y. Karlan, Wolf-Ruprecht Wiedemeyer. Cell cycle requirements shape ovarian cancer progression. [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 1749. doi:10.1158/1538-7445.AM2013-1749

Indole-3-carbinol synergistically sensitises ovarian cancer cells to bortezomib treatment

BACKGROUND

Bortezomib is a proteasome inhibitor with minimal clinical activity as a monotherapy in solid tumours, but its combination with other targeted therapies is being actively investigated as a way to increase its anticarcinogenic properties. Here, we evaluate the therapeutic potential of co-treatment with bortezomib and indole-3-carbinol (I3C), a natural compound found in cruciferous vegetables, in human ovarian cancer.

METHODS

We examined the effects of I3C, bortezomib and cisplatin in several human ovarian cancer cell lines. Synergy was determined using proliferation assays and isobologram analysis. Cell cycle and apoptotic effects were assessed by flow cytometry. The mechanism of I3C and bortezomib action was determined by RNA microarray studies, quantitative RT-PCR and western blotting. Antitumour activity of I3C and bortezomib was evaluated using an OVCAR5 xenograft mouse model.

RESULTS

I3C sensitised ovarian cancer cell lines to bortezomib treatment through potent synergistic mechanisms. Combination treatment with bortezomib and I3C led to profound cell cycle arrest and apoptosis as well as disruptions to multiple pathways, including those regulating endoplasmic reticulum stress, cytoskeleton, chemoresistance and carcinogen metabolism. Moreover, I3C and bortezomib co-treatment sensitised ovarian cancer cells to the standard chemotherapeutic agents, cisplatin and carboplatin. Importantly, in vivo studies demonstrated that co-treatment with I3C and bortezomib significantly inhibited tumour growth and reduced tumour weight compared with either drug alone.

CONCLUSION

Together, these data provide a novel rationale for the clinical application of I3C and bortezomib in the treatment of ovarian cancer.

Abstract 3880: The natural dietary phytochemical indole-3-carbinole (I3C) sensitizes ovarian cancer cells to the proteasome inhibitor bortezomib through inhibition of cyclin E activity

Introduction: Cyclin E deregulation appears to be an important event in the pathogenesis of a subset of epithelial ovarian cancers associated with poor outcome. One mechanism of cyclin E deregulation is through enhanced p27 degradation, which eliminates a powerful negative regulator of cyclin E. The proteasome inhibitor bortezomib has been shown to inhibit the growth of both ovarian and colorectal tumor cell lines through upregulation of p27 and induction of apoptosis, giving it a potential therapeutic role in the subset of ovarian cancers that overexpress cyclin E. Studies demonstrate that as many as five low molecular weight (LMW) isoforms of cyclin E exist, while only the 50-kDa cyclin E form is typically expressed in normal tissues. These LMW isoforms are tumor-specific and cause increased cell proliferation, elevated kinase activity and increased clonogenicity. LMW cyclin E isoforms are generated via proteolysis of the normal 50-kDa cyclin E form by elastase, which itself can be selectively inhibited by indole-3-carbinol (I3C), a natural component of Brassica vegetables. I3C exhibits potent anticarcinogenic properties and has recently been shown to shift the stable accumulation of cyclin E from the LMW to 50-kDa cyclin E form. By taking advantage of the specific inhibitory properties of I3C and bortezomib in the processing and potential expression of cyclin E, respectively, we hypothesize that ovarian cancers overexpressing cyclin E may demonstrate an enhanced response to targeted combination therapy with I3C and bortezomib.

Methods: A panel of ovarian cancer cell lines was screened for cyclin E and p27 protein levels through western blotting. Representative cell lines with high and low cyclin E expression were treated with I3C and bortezomib and evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell proliferation assay and western blotting analysis.

Results: OVCAR3 was found to express high levels of cyclin E and moderate levels of p27. In contrast, OVCAR5 was found to express low levels of cyclin E and high levels of p27. Treatment of OVCAR3 cells with I3C stabilized the expression of the normal 50-kDa cyclin E form in a dose-dependent manner. The combination of I3C and bortezomib treatment in OVCAR3 and OVCAR5 cells caused a significantly greater degree of cytotoxicity compared to either drug alone. While the effect was seen in both cell lines, inhibition of cell proliferation was more enhanced in the cyclin E overexpressing cell line OVCAR3.

Conclusions: Indole-3-carbinol is a natural dietary substance that sensitizes ovarian cancer cells to the proteasome inhibitor bortezomib through stabilization and inhibition of cyclin E activity.

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 3880.

Drosophila E2F1 has context-specific pro- and antiapoptotic properties during development

E2F transcription factors are generally believed to be positive regulators of apoptosis. In this study, we show that dE2F1 and dDP are important for the normal pattern of DNA damage-induced apoptosis in Drosophila wing discs. Unexpectedly, the role that E2F plays varies depending on the position of the cells within the disc. In irradiated wild-type discs, intervein cells show a high level of DNA damage-induced apoptosis, while cells within the D/V boundary are protected. In irradiated discs lacking E2F regulation, intervein cells are largely protected, but apoptotic cells are found at the D/V boundary. The protective effect of E2F at the D/V boundary is due to a spatially restricted role in the repression of hid. These loss-of-function experiments demonstrate that E2F cannot be classified simply as a pro- or antiapoptotic factor. Instead, the overall role of E2F in the damage response varies greatly and depends on the cellular context.

p55, the Drosophila ortholog of RbAp46/RbAp48, is required for the repression of dE2F2/RBF-regulated genes

Many proteins have been proposed to be involved in retinoblastoma protein (pRB)-mediated repression, but it is largely uncertain which cofactors are essential for pRB to repress endogenous E2F-regulated promoters. Here we have taken advantage of the stream-lined Drosophila dE2F/RBF pathway, which has only two E2Fs (dE2F1 and dE2F2), and two pRB family members (RBF1 and RBF2). With RNA interference (RNAi), we depleted potential corepressors and looked for the elevated expression of groups of E2F target genes that are known to be directly regulated by RBF1 and RBF2. Previous studies have implicated histone deacetylase (HDAC) and SWI/SNF chromatin-modifying complexes in pRB-mediated repression. However, our results fail to support the idea that the SWI/SNF proteins are required for RBF-mediated repression and suggest that a requirement for HDAC activities is likely to be limited to a subset of targets. We found that the chromatin assembly factor p55/dCAF-1 is essential for the repression of dE2F2-regulated targets. The removal of p55 deregulated the expression of E2F targets that are normally repressed by dE2F2/RBF1 and dE2F2/RBF2 complexes in a cell cycle-independent manner but had no effect on the expression of E2F targets that are normally coupled with cell proliferation. The results indicate that the mechanisms of RBF regulation at these two types of E2F targets are different and suggest that p55, and perhaps p55's mammalian orthologs RbAp46 and RbAp48, have a conserved function in repression by pRB-related proteins.

Native E2F/RBF complexes contain Myb-interacting proteins and repress transcription of developmentally controlled E2F target genes

The retinoblastoma tumor suppressor protein (pRb) regulates gene transcription by binding E2F transcription factors. pRb can recruit several repressor complexes to E2F bound promoters; however, native pRb repressor complexes have not been isolated. We have purified E2F/RBF repressor complexes from Drosophila embryo extracts and characterized their roles in E2F regulation. These complexes contain RBF, E2F, and Myb-interacting proteins that have previously been shown to control developmentally regulated patterns of DNA replication in follicle cells. The complexes localize to transcriptionally silent sites on polytene chromosomes and mediate stable repression of a specific set of E2F targets that have sex- and differentiation-specific expression patterns. Strikingly, seven of eight complex subunits are structurally and functionally related to C. elegans synMuv class B genes, which cooperate to control vulval differentiation in the worm. These results reveal an extensive evolutionary conservation of specific pRb repressor complexes that physically combine subunits with established roles in the regulation of transcription, DNA replication, and chromatin structure.

Distinct mechanisms of E2F regulation by Drosophila RBF1 and RBF2

RBF1, a Drosophila pRB family homolog, is required for cell cycle arrest and the regulation of E2F-dependent transcription. Here, we describe the properties of RBF2, a second family member. RBF2 represses E2F transcription and is present at E2F-regulated promoters. Analysis of in vivo protein complexes reveals that RBF1 and RBF2 interact with different subsets of E2F proteins. dE2F1, a potent transcriptional activator, is regulated specifically by RBF1. In contrast, RBF2 binds exclusively to dE2F2, a form of E2F that functions as a transcriptional repressor. We find that RBF2-mediated repression requires dE2F2. More over, RBF2 and dE2F2 act synergistically to antagonize dE2F1-mediated activation, and they co-operate to block S phase progression in transgenic animals. The network of interactions between RBF1 or RBF2 and dE2F1 or dE2F2 reveals how the activities of these proteins are integrated. These results suggest that there is a remarkable degree of symmetry in the arrangement of E2F and RB family members in mammalian cells and in DROSOPHILA.