Ingle, JN

Abstract Background: Estrogen receptor beta (ERβ), unlike ERα, classically functions as a tumor suppressor in vitro. However, ERβ's biological functions in vivo and predictive/prognostic value in breast cancer are controversial. Methods: Expression of ERβ protein was determined using a well characterized and validated ERβ specific monoclonal antibody that only recognizes the full-length form of this receptor (PPG5/10) in the following 3 cohorts: 1) a cohort with all breast cancer subtypes (n = 182), 2) a prospective NCCTG adjuvant tamoxifen trial for postmenopausal women with ERα positive breast cancer with long-term follow-up (n = 198) and 3) a cohort of 80 triple negative breast cancers (TNBCs). To elucidate the biological functions of ERβ in breast cancer, novel ERβ expressing MCF7 and MDA-MB-231 cell lines were developed and characterized using multiple techniques and were examined for responsiveness to various ERβ targeted therapies. Results: About one-third of all breast tumors, regardless of sub-type, were shown to express nuclear ERβ and this expression was independent of ERα or HER2. In the NCCTG 89-30-52 cohort, breast cancer recurrence rates were significantly associated with ERβ protein expression with 10 year recurrence rates of 25%, 15% and 4% for zero, low or high levels of ERβ expression respectively. Interestingly, in TNBCs, nuclear ERβ was expressed at intermediate or high levels in 24% of tumors. In the triple negative MDA-MB-231 cell line, expression of ERβ led to inhibition of proliferation and induction of apoptosis in response to estrogen and multiple ERβ specific agonists. Conversely, these same treatments induced proliferation of ERβ-expressing MCF7 cells which endogenously express ERα. However, ERβ expression sensitized MCF7 cells to the anti-proliferative effects of anti-estrogens. Microarray analysis and RT-PCR profiling of MDA-MB-231-ERβ cells revealed that estrogen and ERβ agonists highly induced the expression of multiple cystatins, a family of small secreted cysteine protease inhibitors which function as tumor suppressors, and potently inhibited canonical TGFβ signaling. Conditioned media isolated from estrogen or ERβ agonist treated MDA-MB-231-ERβ cells suppressed the proliferation rates and inhibited TGFβ signaling in other TNBC cell lines, effects that were completely reversed following the depletion of cystatins from the conditioned media. Conclusions: These data demonstrate that ERβ is expressed in a substantial proportion of breast cancers and may have value as a predictive and/or prognostic biomarker. Therapeutic targeting of ERβ may have clinical benefit in multiple breast cancer sub-types; however, the specific drug of choice may vary based on ERα status. Specifically, we have demonstrated that ERβ expression in ERα+ MCF7 cells sensitizes them to the effectiveness of anti-estrogens, an observation that was confirmed in women enrolled in a prospective adjuvant tamoxifen trial. In TNBCs, where targeted therapies are lacking, our data suggest that targeting ERβ with either estrogen or ERβ specific agonists will elicit anti-tumor effects through the induction of cystatins and inhibition of TGFβ signaling resulting in tumor regression and improved patient outcomes. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P6-04-03.

Abstract Background: Commonly used endocrine therapies for breast cancer, such as aromatase inhibitors in postmenopausal women and tamoxifen in premenopausal women, have deleterious effects on bone mineral density. Therefore, the identification of novel cancer therapies which either maintain or improve bone mass are of clinical need. We and others have previously demonstrated that endoxifen is the most active tamoxifen metabolite responsible for eliciting the anti-cancer effects of this drug and that endoxifen concentrations are an important factor with regard to tamoxifen efficacy. These studies have led to the development of endoxifen as a novel anti-breast cancer drug for which phase I clinical trials are now underway. At present, there are no data regarding endoxifen's effects on bone. Methods: The effects of endoxifen on osteoblast gene expression profiles were compared to that of estrogen, tamoxifen, raloxifene and lasofoxifene by microarray and RT-PCR analyses in both estrogen receptor alpha (ERα) and ERβ expressing cell lines. The in vivo effects of an anti-cancer dose of endoxifen (50mg/kg/day) on the skeleton were first analyzed in 3-month-old ovariectomized C57BL/6 mice using Dual-energy X-ray absorptiometry, peripheral Quantitative Computed Tomography, micro-Computed Tomography and histomorphometry. In a second set of studies, a pre-clinical rat model was used to determine the effects of endoxifen (10mg/kg/day) on the skeleton in both a pre- and post-menopausal setting. Results: Endoxifen treatment of ERα and ERβ expressing mouse osteoblast cells led to dramatically different gene expression profiles when compared to that of estrogen and other anti-estrogens. In ovariectomized mice, daily administration of endoxifen led to significant increases in bone mineral density and content throughout the skeleton relative to vehicle control treated animals. The numbers and activity of both osteoblasts and osteoclasts were also found to be significantly higher in endoxifen treated mice. In the pre-clinical model system, endoxifen treatment of 4 month-old ovariectomized Sprague-Dawley rats significantly protected against bone loss following estrogen depletion primarily due to suppression of osteoclast mediated bone resorption. Importantly, in sham operated rats (thus retaining ovarian function), endoxifen treatment enhanced bone volume and trabecular thickness and did not suppress osteoclast activity. Conclusions: These data are the first to examine the effects of the novel breast cancer therapy, endoxifen, on bone and reveal that the molecular mechanisms of action of this compound are substantially different than that of other SERMs. Endoxifen was shown to protect against bone loss following estrogen depletion in both mice and rats and interestingly, enhanced bone mass in ovary intact rats, an observation that is in stark contrast to the known effects of tamoxifen which induces bone loss in the “pre-menopausal” setting. These studies suggest that endoxifen may have superior bone-beneficial effects compared to tamoxifen, and if efficacy is confirmed in later phase trials, endoxifen may represent a better drug of choice for a sub-set of breast cancer patients. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-05-01.

Abstract BACKGROUND: Breast cancer affects thousands of women each year. Epithelial to Mesenchymal transition (EMT) has been associated with increased metastatic potential of cancer cells as well as resistance to chemotherapy. Predicatively, presence of EMT leads to worse prognosis in patients with breast cancer. We have recently showed that Aurora-A plays a key role in development of EMT and increased ability of breast cancer cells for self renewal. Therefore we hypothesized that inhibition of Aurora-A in breast cancer cells will lead to increased sensitivity to chemotherapy. RESULTS: Increased Aurora-A expression, as assessed by IHC staining, in patients with locally advanced disease was associated with worse prognosis. We show that in ER+ cell line, MCF7, activating MAPK pathway through overexpression of Raf1 leads to increase in Aurora-A activity. These cells have decreased sensitivity to treatment with paclitaxel when compared to untransfected cells in MTT assays. This effect is even more exacerbated with over-expression of Aurora-A. The sensitivity to paclitaxel was restored with inhibition of Aurora-A by specific inhibitor, Alisertib. This was further explored in triple negative (MDA-MB-231) or Her-2/neu expressing (SUM149) cell lines. These cells express higher levels of Aurora-A. Inhibition of Aurora-A activity in combination with paclitaxel was superior when compared to either therapy alone. Similar effect was seen with use of anthracyclines. Inhibition of Aurora-A resulted in decreased SMAD5 expression as well as decreased Akt phosphorylation. Current studies are looking at a role of Aurora-A in activating SMAD5 and its role in resistance to chemotherapy. In vivo experiments evaluating combination therapies in breast cancer animal model are ongoing. CONCLUSIONS: Inhibition of Aurora-A by a specific inhibitor, MLN8237, resulted in decreased SMAD5 expression and increased effectiveness of chemotherapeutic agents in breast cancer cells. These results contribute to better understanding of signaling pathways involved in resistance of breast cancer cells to chemotherapy. This knowledge could be very useful in developing more effective treatments for breast cancer patients in neo-adjuvant, adjuvant and metastatic settings. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-09-07.

Abstract Background- The ability to consistently detect cell-free tumor-specific DNA in peripheral blood of patients with metastatic breast cancer provides the opportunity to detect changes in tumor burden and to monitor response to treatment. Studies of cell-free DNA in the peripheral blood of breast cancer patients suggest that methylated DNA markers in serum or plasma could be used for detection of advanced disease, monitoring of therapeutic response, and for early detection of disease recurrence. Methods- A genome-wide serum DNA methylome array (Illumina HumanMethylation27 BeadChip) analysis was conducted on cell-free circulating DNA in serum from women with stage IV recurrent breast cancer, and 232 key CpG loci were identified. Methylation for this panel of 10 gene loci was evaluated using our newly developed cMethDNA assay to detect miniscule amounts of methylated DNA in Training and Test sets of sera from a total of 112 women (n = 55 normal, n = 57 metastatic breast cancer). The clinical sensitivity and specificity of the assay, along with technical reproducibility, was determined. To evaluate the concordance of DNA methylation patterns, the 10 gene panel was tested on 22 DNA sets of primary tumor, metastases and serum from the same patient. Finally, the ability of cMethDNA to monitor response to therapy was evaluated in 28 patients with metastatic disease. Results- A normal laboratory threshold of 7 cumulative methylation units was set and assay parameters were locked, based on Receiver Operating Characteristic (ROC) analyses of DNA from 300 ul of patient sera in the Training set (normal, n = 28; cancer, n = 24; 92% sensitivity, 96% specificity, and AUC = 0.950). Evaluation of the Test set of patient sera (normal, n = 27; cancer n = 33) resulted in detection of metastatic breast cancer with 91% sensitivity, 100% specificity, and AUC = 0.994 (0.984-1.005, p<0.0001). Reproducibility of the cMethDNA assay increased with copy number; with the highest variation at 50 copies (CV = 29.1%) and the lowest at 3,200 copies (CV = 2.5%) of methylated DNA. The test was shown to be operator independent (ICC = 0.99). Evaluation of concordance between primary and disseminated tumor methylation showed that the methylation pattern from any given individual is highly conserved between serum, primary tissue and their metastases, and poorly conserved between different individuals. cMethDNA analysis of 28 patients before and after initiation of therapy showed a decrease in cumulative methylation in women with stable/responsive disease and a correlation with disease progression free survival (p<0.0001). Conclusion- Together, our data suggest that the cMethDNA test 1) can detect tumor DNA shed into blood, 2) reflect the methylation alterations typical of the primary tumor and its metastatic lesions, and 3) reflect response to treatment after chemotherapy. Next, we will test the clinical utility of cMethDNA in independent clinical trial sample sets where it's complementary and independent roles will be examined against CA15.3 and CTC assays. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P2-06-01.

Abstract Background: Based upon the association between pathologic response and disease free survival, the neoadjuvant setting is increasingly being used for drug development. NGS has identified unique and recurrent genetic alterations in breast cancer (BC) that are potentially targetable; however, the clinical implications are mostly unknown. We developed a prospective neoadjuvant study (BEAUTY) in high risk BC patients (pts) using weekly T followed by anthracycline-based chemo wherein percutaneous tumor biopsies (PTB) are obtained before/during/after chemo for NGS and PDX. Our goal is to identify novel biomarkers/pathways and develop PDX to test new therapeutic approaches. Methods: Pts underwent PTB at baseline and after 12 wks of T. Response to T was defined based upon 12 week Ki-67: responder (<15%) vs non-responder (≥15%). Pts with histologic response and absence of invasive BC at 12 wks were classified as responders. NGS was performed using PTB/blood DNA (exome) and PTB (RNA seq). MRI response was classified using RECIST criteria. NGS data were used to identify somatic copy number variants (cnvs) and expressed single nucleotide variants (eSNVs). Non-SCID mice (estrogen supplemented) were implanted ≤ 30 minutes with PTB samples. Results: Of the first 78 pts, 44 have completed T. Here we focus on 18 pts with either triple negative or luminal B BC. Clinical characteristics according to Ki-67 response are shown in Table 1. Comparison of genomic alterations in BEAUTY pts with TCGA identified a greater overlap with copy number gains (73%) compared to deletions (40%), along with similar observations of mutations in TP53, PTEN, RYR2, and AKT1 genes. Association analysis of CNVs and eSNVs between responders/non-responders identified 33 genes (predominantly located in chromosomes 1, 8, 13) and 580 eSNVs (corresponding to 497 genes) with a p < 0.05. Differential gene expression (DGE) analysis of responders/non-responders identified 198 genes with a p-value < 0.05. Integrated analysis of 539 genes (CNVs, eSNVs and DGE) identified pathways such as TGF-beta, Jak-Stat, WNT and NOTCH signalling. PDX take rate was 44% [triple negative (6/10); Luminal B (2/8)]. PDX growth rate was significantly associated with clinical baseline Ki-67 (p = 0.00014). Conclusion: This is the first prospective study to demonstrate the feasibility of using PTB to obtain both NGS data and PDX in the neoadjuvant setting. PDX take rate is associated with BC subtype and baseline Ki-67. Studies are ongoing to 1) validate genes/pathways associated with treatment response in subsequent BEAUTY pts; 2) genomically characterize and assess PDX in vivo response to T and 3) Use NGS data to prioritize new drugs/drug combinations in PDX. Funded by Mayo Clinic Center for Individualized Medicine and MC Cancer Center. Clinical CharacteristicsOverallResponders: 12 week Ki-67 < 15% (n = 9)Non-Responders: 12 week Ki-67 ≥ 15% (n = 9)Median Age495345T stage T2/T314 (78%)7 (78%)7 (78%)Node Positive8 (44%)4 (44%)4 (44%)Triple negative10 (56%)6 (67%)4 (44%)Luminal B8 (44%)3 (33%)5 (56%)Ki-67 after 12 Weeks of T Median 5% (0-11%)Median 35% (17-60%)Complete/Partial MRI Response after T 6 (67%)2 (22%) Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-08-10.

Abstract Background: Exemestane, a steroidal aromatase inhibitor, reduced the incidence of invasive breast cancers by 65% among 4560 post-menopausal randomized to exemestane or placebo for 5 years on MAP.3. Differences in quality of life (QOL) were judged to be minimal, but only summary information was reported. Purpose: To provide more detailed information about effects of exemestane on menopause-specific and health-related qualities of life. Method: Participation in quality of life assessment was an eligibility criterion. Menopause-specific and health-related qualities of life were assessed using the MENQOL (4 scales; physical, vasomotor, psychosocial, sexual) and SF-36 (8 scales; physical health, role function — physical, bodily pain, general health, vitality, social function, role function — emotional, mental health, and 2 summary scales) instruments, respectively at baseline, 6 months and then yearly after randomization. Compliance with QOL questionnaire completion at each follow-up visit ranged from 93–98%, and did not differ by group. Change scores for each MENQOL and SF-36 scale, calculated for each assessment time relative to baseline, were compared using the Wilcoxon Rank-Sum test. Summary scores were used to summarize the QOL scores observed at each time point for each SF-36 dimension and overall mental (MCS) and physical component summaries (PCS) and MENQOL domains. Clinically important worsening of MENQOL change scores was defined as an increase of ≥0.5/8 points. SF-36 change scores were considered worsened if scores decreased by ≥ 5 points from baseline. Results: Both groups were balanced on scores for MENQOL and SF-36 at baseline. Median follow-up was 35 months and the proportion of women who stopped study medication early for toxicity reasons was 15% in the exemestane arm and 11% in the placebo arm. There was a statistically significant difference in change scores for vasomotor symptoms among women on exemestane during the first 4 years (p-values <0.01), compared to placebo. However, no between-group differences in vasomotor change met the criterion for clinical importance. Women on exemestane had statistically poorer sexual functioning (mean change = −0.02, SD=1.37) compared to placebo (mean change = −0.12, SD=1.32) during the first 6 months on study (p-value = 0.03) but the differences were not statistically significant thereafter or clinically important at any time. Among the 8 SF-36 scales, only bodily pain was statistically different between exemestane and placebo for the first 24 months on study medication (p-value <0.01), but no between-group difference in change scores exceeded 5 points. Overall SF-36 PCS and MCS assessing changes in overall physical and mental health-related QOL did not differ significantly by group at any assessment. Conclusion: Our assessment that early differences in vasomotor symptoms and pain were probably not clinically important is supported by the observation of no between-group differences when overall physical and mental health-related QOL changes were compared. Exemestane does not appear to have a major negative impact on the quality of life among these women. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr S6-1.

Abstract Background: Prediction of benefit from trastuzumab in patients (pts) with HER2+ breast cancer remains an important goal. We sought to investigate the predictive value of quantitative measurement of HER2, HER3, HER4, EGFR, ER and PTEN protein expression on the benefit of trastuzumab in the phase III HER2+ adjuvant N9831 study for pts randomized to chemotherapy alone (Arm A) or chemotherapy with sequential (Arm B) or concurrent trastuzumab (Arm C). Methods: For each marker, we evaluated quantitative expression, relationship with demographic data, and association with disease-free survival (DFS) of pts. Freshly cut tissue microarray slides with up to three-fold redundancy per specimen from the N9831 cohort were treated identically using the AQUA (Camp, et al; Nat Med 2002, JCO 2008) method of quantitative immunofluorescence for each marker. HER2 was tested with CB11 (mouse monoclonal, Biocare, Inc.) and preliminary results were available for 698 of nearly 1400 pt specimens to be tested. The minimum value per pt was used in statistical analysis. Specimens were classified with high versus low expression based on a median value cutpoint for each marker. Median follow-up was 7.0 yrs. Results: Quantitative HER2 was compared with centrally performed HER2 testing by IHC and FISH. Median quantitative HER2 via AQUA was 10,017 units for the HER2 IHC 3+ group (n=607) versus 1058, 831, and 970 for the HER2 IHC 2+ (n=68), 1+ (n=11), and 0 (n=11) groups, respectively. The Spearman correlation between quantitative HER2 and FISH HER2/CEP17 ratio was 0.32 (p<0.001). High quantitative HER2 was associated with lower percentage of hormone receptor positivity (48% vs 59%, chi-sq p=0.003) but not associated with age, race, nodal positivity, tumor histology, grade, or size. High HER2 did not impact DFS in any arm of the study (See Table). Data for additional HER2 testing, HER3, HER4, EGFR, ER and PTEN are in process and will be ready by September, 2011. Conclusions: Similar to results based on standard HER2 testing by IHC and FISH in N9831, quantitative HER2 did not impact benefit from adjuvant trastuzumab. Results for additional markers will be presented. Our complete quantitative results for a second epitope on HER2, HER3, HER4, ER and EGFR will be the first report of these markers in a large patient cohort in the adjuvant setting. Disease Free Survival Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD05-03.

Abstract Background: Based on recent breast cancer literature, we hypothesized that there could be substantive differences in apparent efficacy estimates using a log-normal (LN) survival model rather than with standard Kaplan-Meier (K-M) or Cox model methods. While both Cox and LN survival analyses offer greater specification by individual patient characteristics, the LN model may more robustly estimate survival under model misspecification. Methods: We recently pooled data for 5 NCIC CTG primary breast cancer trials: MA.5, MA.12, MA.14, MA.21, and MA.27. The total patient count for patients who received at least 1 dose of trial therapy is 11,253. Compilation included definition of STEEP endpoints (C Hudis, JCO, 2008) and standardized factor categorizations. The primary endpoint is Breast Cancer Free Interval (BCFI) defined as the time from randomization until recurrence: first local invasive or DCIS; regional, or distant; contralateral invasive or DCIS; or death from breast cancer. We found substantive evidence of non-proportionality for 7 factors compiled for the meta-analyses. In this work, we fit multivariate Cox and LN models with these 7 factors, lymph node status and pathologic T status. We then compare BCFI efficacy estimates for patient and tumour characteristics at 1-, 3-, and 5-years obtained with K-M, Cox, and LN models. Results: There was evidence that the Cox assumption of proportional hazards was violated for 7 factors: age, menopausal status, hormone receptor status, anthracycline use, chemotherapy use, race, and ECOG performance status. Differences between models were intrinsically affected by timing and extent of non-proportionality; there was no consistent pattern. In particular, investigations to date indicate efficacy estimates with absolute differences between K-M, Cox and LN estimates which varied by time of assessment: at 1-year 0.0 to 6.7%, at 3-years 0.4 to 18.6%, and at 5-years 0.2 to 17.0%. BCFI estimates with the K-M were inconsistently closer to those with the LN or Cox model: for K-M to Cox at 1-year 0.4 to 5.2%, at 3-years 0.4 to 15%, at 5-years 0.4 to 14.3%; for K-M to LN at 1-year 0.0 to 6.7%, at 3-years 0.5 to 18.6%, at 5-years 0.2 to 17.0%; for Cox to LN at 1-year 0.8 to 1.8%, at 3-years 1.9 to 6.0%, at 5-years 0.6 to 5.7%. K-M and Cox models have step-wise adjustments at events for K-M and Cox, rather than smooth modeling with the LN. Discussion: Even with reasonably large population subgroups, there were substantive differences in apparent survival (0.0 to 18.6%) between K-M, Cox and LN model types. The magnitude of differences in survival estimates was large enough to be clinically relevant and warrant further consideration as we evaluate new therapies and prognostic/predictive factors. We will be statistically investigating framework robustness under differing levels of model misspecification. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P5-14-01.

Abstract Background Exemestane significantly reduced invasive and preinvasive breast cancers in postmenopausal women at increased risk for breast cancer in the NCIC CTG MAP3 trial with no serious toxicities, including excess fractures or osteoporosis. Purpose: To provide additional information on the effect of exemestane on bone loss in women at high risk for breast cancer, within a subset of women participating on the NCIC CTG MAP.3B study. The primary hypothesis is that exemestane does not induce clinically significant bone loss in postmenopausal women at increased risk of developing breast cancer at 2 years. The primary objective of this companion study is to examine the effect of exemestane on lumbar spine and total hip BMD by DEXA at 2 years in women participating in the MAP3 trial. Methods: The MAP.3B bone sub-study registered women from the main MAP. 3 trial from May 2008 to March 2010. Eligible women had to have an acceptable quality BMD scan by DEXA taken within 12 months prior to randomization to MAP.3. A BMD T-score >-2.0 SD (i.e. better than 2 standard deviations below the average peak BMD of a young adult woman) was established as the study population cutoff. A questionnaire including information on height, falls, fractures, lifestyle information including physical activity, tobacco and alcohol use was completed at baseline, 12 months, 24 months and at last visit. Fasting serum for bone biomarkers was collected at 12 months and total hip and L1-L4 (postero-anterior) spine BMD were measured 2 years after randomization on the same Lunar or Hologic scanner. The primary objective was to determine differences in hip and spine BMD at 2 years. Secondary outcomes include number of skeletal fractures and development of osteoporosis 2 years after randomization and changes in bone biomarkers at 1 year after randomization. For the analysis of the primary endpoints, the upper limit of a one sided 95% confidence interval for the difference in mean percentage changes between placebo and exemestane will be calculated for the BMD by DEXA at each site. We will conclude that exemestane does not induce significant bone loss in postmenopausal women at increased risk of developing breast cancer at 2 years when the upper limit is less than 3% for both sites. Similar confidence interval approach will be used to analyze the secondary endpoints. Results: Between May 2008 and March 2010, 238 postmenopausal women were recruited. Median age was 61.8 years, and the majority of women were Caucasian (91%), with approximately 20% of the participants reporting a recent fall (within past 12 months) and another 13% reporting a recent fracture prior to randomization. We will report results from the primary as well as the secondary endpoints at the SABCS meeting. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-11-13.

Abstract Background The role of estrogen receptor alpha (ERα) in breast cancer has been studied extensively; yet, much less is known about full-length ERβ (ERβ1) and even less about its 4 variant forms (ERβ2-5). We have recently implicated a role for ERβ1 in sensitizing ERα expressing breast cancer cells to anti-estrogens. However, the ability of ERβ2-5 to modulate ERα and ERβ1 activity, and their association with cancer development, progression, and response to estradiol (E2) and anti-estrogens are not well understood. Here, we provide evidence that the presence of ERβ variants may be of diagnostic and clinical relevance for breast cancer patients and describe the development and characterization of a novel, highly specific monoclonal antibody (MC10) that is able to detect their expression in tumor biopsies. Methods: Transient transfection and luciferase assays were used to determine the transcriptional activity of ERβ2-5 in response to E2 and anti-estrogens alone or in combination with ERα and ERβ1. A novel monoclonal antibody targeting all ERβ variants (MC10) was developed and characterized. The sub-cellular localization of ERβ2-5 was determined via confocal microscopy. Finally, the MC10 antibody was used to assess ERβ positivity in breast tumors and was compared to that of another monoclonal antibody which only detects ERβ1. Results: Unlike ERβ1, ERβ2-5 do not activate an estrogen response element (ERE) in response to E2 and instead, slightly repress the activity of this reporter construct. Expression of ERβ2-5 does not significantly alter the transcriptional activity of ERβ1 following E2 treatment. However, ERβ2, 3 and 5, but not ERβ4, significantly enhance the E2-induced transcriptional activity of ERα. Interestingly, expression of ERβ3, 4 and 5, but not ERβ2, enhance the ability of anti-estrogens to block ERα mediated transcriptional activity. Confocal microscopy revealed that ERβ1 and 2 are almost exclusively localized to the cell nucleus. However, ERβ3-5 exhibit significant cytoplasmic and peri-nuclear localization. Immunohistochemistry of breast cancer biopsies using the MC10 antibody revealed multiple staining patterns including tumors which exhibit primarily nuclear staining and others primarily cytoplasmic, both in the presence and absence of ERα. These results are in contrast to the almost exclusive nuclear staining obtained on the same tumors with an ERβ1-specific antibody. Conclusions: ERβ variants exhibit variable sub-cellular localization patterns and can influence the function of ERα, both in response to E2 and anti-estrogens. Therefore, the differential expression of ERβ variants and their cellular localization may influence breast cancer progression and/or therapeutic responses. The use of ERβ antibodies which do not detect all ERβ variants, or the use of a single ERβ antibody which does not discriminate between ERβ1 and its variants, is unlikely to reveal the complete biological significance of total ERβ expression in breast cancer and may in part explain the conflicting studies which have been reported for ERβ in the field. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-02-03.