Multiscale analysis of Klf10's impact on the passive mechanical properties of murine skeletal muscle

Skeletal muscle is a hierarchical structure composed of multiple organizational scales. A major challenge in the biomechanical evaluation of muscle relates to the difficulty in evaluating the experimental mechanical properties at the different organizational levels of the same tissue. Indeed, the ability to integrate mechanical properties evaluated at various levels will allow for improved assessment of the entire tissue, leading to a better understanding of how changes at each level evolve over time and/or impact tissue function, especially in the case of muscle diseases. Therefore, the purpose of this study was to analyze a genetically engineered mouse model (Klf10 KO: Krüppel-Like Factor 10 knockout) with known skeletal muscle defects to compare the mechanical properties with wild-type (WT) controls at the three main muscle scales: the macroscopic (whole muscle), microscopic (fiber) and submicron (myofibril) levels. Passive mechanical tests (ramp, relaxation) were performed on two types of skeletal muscle (soleus and extensor digitorum longus (EDL)). Results of the present study revealed muscle-type specific behaviors in both genotypes only at the microscopic scale. Interestingly, loss of Klf10 expression resulted in increased passive properties in the soleus but decreased passive properties in the EDL compared to WT controls. At the submicron scale, no changes were observed between WT and Klf10 KO myofibrils for either muscle; these results demonstrate that the passive property differences observed at the microscopic scale (fiber) are not caused by sarcomere intrinsic alterations but instead must originate outside the sarcomeres, likely in the collagen-based extracellular matrix. The macroscopic scale revealed similar passive mechanical properties between WT and Klf10 KO hindlimb muscles. The present study has allowed for a better understanding of the role of Klf10 on the passive mechanical properties of skeletal muscle and has provided reference data to the literature which could be used by the community for muscle multiscale modeling.

The Effect of Freezing Time on Muscle Fiber Mechanical Properties

The purpose of this study was to investigate the effect of freezing time on the functional behavior of mouse muscle fibers. Passive mechanical tests were performed on single soleus muscle fibers from fresh (0 month) and preserved (stored at -20°C for 3, 6, 9 and 12 months) 3 month old mice. The Young's modulus and the dynamic and the static stresses were measured. A viscoelastic Hill model of 3rd order was used to fit the experimental relaxation test data. The statistical analysis corresponding to the elastic modulus of single muscle fibers did not differ when comparing fresh and stored samples for 3 and 6 months at -20 °C. From 9 months, fibers were less resistant and the mechanical properties were damaged. The primary goal of this study was to complete the gold standard process of muscle fiber preservation for subsequent mechanical property studies. We have demonstrated that muscle fibers can be stored at -20°C for up to 6 months without altering their mechanical properties.

Abstract P6-19-05: Clinical characteristics and survival of patients with male breast cancer: The Mayo Clinic experience

Background:

Male breast cancer (MBC) is rare, and usually managed by extrapolation from female breast cancer. We report on the characteristics and survival outcomes of MBC patients from Mayo Clinic Rochester (MCR).

Methods:

Medical records of MBC patients treated at MCR during a 25-year period (1990-2015) were reviewed. Demographic variables, tumor characteristics, recurrences, and overall survival (OS) were collected. Progression free survival (PFS) and OS were estimated by the Kaplan-Meier method. Multivariate Cox-proportional hazard regression was used to identify predictors of OS.

Results:

One hundred sixty-seven patients were included in the final analysis, with a median follow-up of 58 months after diagnosis. Baseline characteristics are presented in Table 1. Eighty percent of patients with ER-positive tumors received endocrine therapy. Among men with stage I-III disease, approximately 90% underwent mastectomy, and 44% received adjuvant chemotherapy.

The 5-year locoregional and distant recurrence rates for patients with stage I-III disease were 4.4% and 21.5%, respectively. The 5-year PFS and OS for patients with stage I-III disease were 65.5% and 80.1%, respectively. In a multivariate analysis assessing predictors of OS in patients with stage I-III disease, older age (HR 1.05; 95% CI: 1.02 – 1.09), stage II (HR 11.06; 95% CI: 3.84 – 31.85) or stage III disease (HR 14.74; 95% CI (3.99 – 54.45), and omission of surgery (HR 45.33; 95% CI: 3.97 – 517.32) were associated with poorer OS, while endocrine therapy (HR 0.21, 95% CI: 0.09 – 0.51) was associated with better OS. ER, PR, HER2 and grade were not independently prognostic.

The median OS for stage IV patients was 10 months, though this 11-man cohort was too small to allow assessment of prognostic factors in advanced male breast cancer.

Conclusions:

MBC remains an understudied condition. Prognostic factors in this stage I-III disease are consistent with those identified in other MBC retrospective cohorts. Prospective studies are needed to better understand the unique clinical features of MBC, and to improve outcomes, particularly for advanced disease.

Table 1:Baseline characteristics N=167  Median age at diagnosis (Years)64.4  Ethnicity/Race: Caucasian131 (78.4%)African American4 (2.4%)Other or unknown32 (19.2%)  Overall AJCC 7th edition stage: Stage I39 (23.4%)Stage II80 (47.9%)Stage III32 (19.2%)Stage IV11 (6.6%)Unknown5 (3.0%)  Grade: 18 (4.8%)247 (28.1%)3101 (60.5%)Unknown12 (7.1%)  ER status: Negative8 (4.8%)Positive153 (91.6%)Unknown6 (3.6%)  PR status: Negative17 (10.2%)Positive141 (84.4%)Unknown9 (5.4%)  HER-2 status: Negative70 (41.9%)Positive12 (7.2%)Unknown85 (50.9%)

Citation Format: Yadav S, Leon-Ferre RA, Jimenez RE, Hawse JR, Hieken TJ, Couch FJ, Boughey JC, Ruddy KJ. Clinical characteristics and survival of patients with male breast cancer: The Mayo Clinic experience [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-19-05.

Abstract P5-04-05: Glucocorticoid receptor activation inhibits proliferation of endoxifen resistant breast cancer cells and resensitizes cells to hormonal therapy

Background: Despite the prevalent treatment options for ERα-positive breast cancer patients, and their initial efficacy for many women, ERα-positive disease still accounts for more breast cancer related deaths than any other subtype. Relapse in these patients is largely due to the development of resistance to anti-estrogen therapies such as tamoxifen. While tamoxifen and its resistance mechanisms have been extensively studied from both the bench and the bedside, relatively little is known about its active metabolite endoxifen. Our group has provided evidence that endoxifen is the most potent and clinically relevant metabolite of tamoxifen, suggesting that its characterization may be crucial to understanding tamoxifen resistance.

Methods: We have developed novel endoxifen resistant MCF7 and T47D cell lines through chronic exposure to endoxifen during a period of 12-24 months. Using these models and their respective controls, we compared global gene expression profiles of endoxifen resistant cells to tamoxifen resistant cells and found marked differences between the two models. Additionally, we subjected treatment naïve cells to a genome-wide, CRISPR-mediated knockout screen to identify genes, and their associated pathways, that are likely involved in mediating endoxifen resistance.

Results: Analysis of CRISPR guide RNAs enriched or depleted in response to chronic endoxifen treatment revealed that disruption of genes regulated by dexamethasone (Dex), a potent glucocorticoid receptor (GR) agonist, enhanced cells' ability to survive and proliferate in the presence of endoxifen. These data suggest that GR activation may inhibit endoxifen resistance, and that treatment of resistant cells with Dex may restore endoxifen efficacy. Indeed, Dex treatment significantly inhibited the proliferation rates of endoxifen resistant cells by 50-60% with little to no inhibitory effects in endoxifen sensitive models. Further, Dex was shown to synergize with endoxifen in resistant cells to further suppress cell proliferation, implying that Dex treatment could be utilized as an effective therapy for endocrine resistant disease. Conditioned media harvested from cells chronically exposed to Dex also resulted in substantial inhibition of endoxifen resistant cell proliferation rates. To explore potential mechanisms of these effects, we performed RNA-seq on both treatment-naïve and endoxifen resistant cells following Dex treatment. Out of 246 genes significantly regulated by Dex in endoxifen resistant cells, we identified 61 genes that were not differentially regulated in treatment naïve cells. These genes may provide insights into the mechanisms of GR activity specific to endoxifen resistant cells.

Conclusions: To our knowledge, we have developed the first models of endoxifen resistance and have demonstrated that global transcriptomic changes that occur during this process are substantially different than those observed in tamoxifen resistant models. We have shown that activation of GR signaling elicits significant growth-inhibitory effects specifically in the setting of endoxifen resistance. These data identify the GR pathway as a potential novel therapeutic target for the treatment of endocrine resistant breast cancer.

Citation Format: Jones CJ, Goetz MP, Ingle JN, Hawse JR. Glucocorticoid receptor activation inhibits proliferation of endoxifen resistant breast cancer cells and resensitizes cells to hormonal therapy [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-04-05.

Abstract P5-04-04: Activation of ERβ in triple negative breast cancer results in cell cycle arrest

Background: Triple negative breast cancer (TNBC), which comprises approximately 20% of breast cancer diagnoses, lacks estrogen receptor alpha, progesterone receptor and Her-2 expression. However, we have identified that 30% of TNBC patients express estrogen receptor beta (ERβ), a nuclear hormone receptor and potential therapeutic target. Here we examine the effects of ERβ in triple negative breast cancer cell lines.

Methods: Cell lines that stably express ERβ were used to perform microarray analyses following five days of estrogen treatment. Ingenuity pathway analysis was conducted on differentially expressed genes to determine alterations in biological pathways. The effects of ERβ on cell cycle progression and apoptosis was determined. Cell cycle-related expression changes were confirmed with RT-qPCR and western blotting. The impact of siRNA mediated gene silencing of CDK1 on TNBC cell proliferation was assessed as was the ability of ERβ to elicit anti-proliferative effects in the setting of CDK1 knockdown.

Results: We have shown that estrogen or ERβ-specific agonist treatment causes decreased proliferation of ERβ+ TNBC cells. This inhibitory effect is not due to programmed cell death but rather a G1/S phase cell cycle arrest as indicated by flow cytometry experiments. Microarray data and ingenuity pathway analysis revealed a number of down regulated genes involved in cell-cycle progression. Specifically, estrogen treatment of ERβ positive TNBC cells was shown to result in suppression of cyclin-dependent kinase 1 (CDK1) and Cyclin B, effects that were confirmed following ERβ-specific agonist treatment at both the mRNA and protein levels via RT-qPCR and western blotting, respectively. Knockdown of CDK1 in ERβ+ TNBC cells using siRNA resulted in decreased proliferation and diminished the anti-proliferative effects observed following estrogen or ERβ-specific agonist treatment.

Conclusions: Our data demonstrate that estrogen and ERβ-specific agonists cause cell cycle arrest in ERβ positive TNBC. These effects are due to ERβ-mediated suppression of multiple genes involved in cell cycle progression including CDK1 and Cyclin B. Following knockdown of CDK1, estrogen or ERβ-specific agonist treatment displayed minimal impact on cell proliferation. Therefore, ERβ's effects on proliferation may primarily be mediated by blockade of CDK1 and Cyclin B. Regardless of ERβ, our data suggest that inhibition of CDK1 activity may have therapeutic benefit in a subset of TNBC patients, an area of study that has yet to be explored.

Citation Format: Reese JM, Bruinsma ES, Monroe DG, Goetz MP, Hawse JR. Activation of ERβ in triple negative breast cancer results in cell cycle arrest [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P5-04-04.

Abstract P1-08-03: Identification and characterization of a novel endoxifen substrate, PKCβ1, and its interaction with the estrogen receptor

Background: The primary mechanism by which tamoxifen (Tam) and its metabolites exert their biologic effects is through estrogen receptor (ER) binding and inhibition of ER signaling. We and others demonstrated that endoxifen (Endx) has greater antitumor activity in vitro and in vivo compared to Tam and the first-in-human Endx phase I study demonstrated its antitumor activity in patients with prior progression on Tam (Goetz SABC 2015). PKCs are a family of serine/threonine-specific protein kinases that regulate signaling pathways involved in cell proliferation and tumorigenic transformation. Our prior protein docking studies suggested endoxifen may be a substrate for PKCs. Here we report the effects of Tam and Endx on PKCβ1 binding, kinase activity, as well as interactions between PKCβ1 and ERα.

Methods: Surface Plasmon Resonance (SPR, Biacore T200, GE Healthcare) was used to evaluate binding of Tam, N-desmethyl Tam (NDMT), 4-HT, and Endx to PKCβ1 and PKCβ2. The effects of Tam and Endx on PKCβ1 kinase activity were determined. Proliferation and colony formation in MCF7 parental and PKCβ1 overexpressing cells were evaluated. siRNA silencing was used to knockdown PKCβ1 expression in the following cells: MCF7 aromatase expressing cells that were either sensitive (MCF7/AC1) or resistant to letrozole (MCF7/AC1 L-resistant); T47D; and MDA-MB-361. Coimmunoprecipitation assay and DUOlink in situ proximity ligation were used to investigate the interaction between PKCβ1 and ERα.

Results: Endx more potently inhibited PKCβ1 kinase activity compared to Tam ( IC50 350 nM vs 47.8 μM ) with KDs for PKCβ1 binding as follows: Endx (100 nM ), Tam ( 2 μM ), 4-HT ( 2 μM ) and NDMT (> 7 μM ). None of the SERMs exhibited PKCβ2 binding. In the MCF7/AC1 and MCF7/AC1 L-resistant cells, PKCβ1 knockdown resulted in ERα degradation and potently inhibited cell proliferation. These results were confirmed in T47D and MDA-MB-361 cells. Notably, PKCβ1 knockdown in MCF7/AC1 cells resulted in significantly greater E2 induced proliferation comparing siRNA knockdown vs. control. To further explore these effects, we evaluated the effects of PKCβ1 overexpression in MCF7 cells and demonstrated that PKCβ1 overexpression reduced cell proliferation and colony formation compared to parental MCF-7 cells without affecting ERα protein stability. Coimmunoprecipitation assays in transient transfected MCF-7 cells with exogenous PKCβ1 as well as PKCβ1 expressing MDA-MB-231 cells transiently or stably transfected with ERα demonstrated PKCβ1 and ERα interaction, with confirmation by Duolink assay that this interaction occurs in the cytoplasm.

Conclusions: Our findings demonstrated that endoxifen binds and inhibits PKCβ1 at relevant concentrations achieved in the endoxifen clinical trial studies. PKCβ1 interacts with cytoplasmic ERα and PKCβ1 knockdown inhibits cell proliferation and enhances ERα turnover. However, in PKCβ1 overexpressing cells, PKCβ1 may exhibit tumor suppressive effects. These data suggest a complex interaction between PKCβ1 and ERα and that endoxifen's effects on PKCβ1 may alter drug response of endocrine therapy. Further studies are ongoing to characterize the role of PKCβ1 and its role in ER biology and response to endoxifen.

Citation Format: Guo C, Kuffel MJ, Kudgus RA, Huang Z, Bode AM, Cheng J, Suman VJ, Reid JM, Bruinsma ES, Subramaniam M, Ames MM, Hawse JR, Goetz MP. Identification and characterization of a novel endoxifen substrate, PKCβ1, and its interaction with the estrogen receptor [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-08-03.

Abstract P3-07-20: Biological functions of ERβ in triple negative breast cancer and its utility as a novel therapeutic drug target

Background: Triple negative breast cancer (TNBC) accounts for approximately 20% of all breast cancer diagnoses. It is the most aggressive form of breast cancer and clinical management is problematic due to lack of available targeted therapies. We have shown that approximately 30% of all TNBCs express estrogen receptor beta (ERβ), a ligand binding transcription factor, and a potential drug target for patients with this form of the disease.

Methods: Using novel ERβ-expressing TN cell lines developed in our laboratory, we assessed the impacts of ERβ on proliferation, invasion, migration, and alterations in cell cycle progression following estrogen and ERβ-specific agonist treatment. We also characterized the ERβ transcriptome and cistrome in these models through microarray and ChIP-Seq, respectively. Finally, we determined the tumoral response of cell line xenografts and PDXs treated with 17β-estradiol.

Results: We found that both estrogen and multiple ERβ-specific agonists elicit significant anti-tumor effects in ERβ+ TNBC cell lines and tumor xenografts. Activation of ERβ with estrogen and ERβ-specific agonists resulted in inhibition of cell proliferation primarily through a G1/S phase cell cycle arrest. Substantial reductions in cell migration and invasion were also observed following treatment. Microarray studies revealed that ERβ differentially regulated the expression of approximately 1000 genes following estrogen treatment. Of these genes, the most striking effects were observed in a family of small secreted cysteine protease inhibitors known as cystatins, which were highly induced following ERβ activation. ChIP-Seq and ChIP-PCR identified ERβ binding sites in the promoter region of each cystatin and demonstrated ERβ-mediated alterations in chromatin marks and recruitment of PolII around these promoters. We found that cystatins directly interact with TGFβ receptor 2 (TGFβR2) and block downstream TGFβ ligand-mediated activation of the canonical signaling pathway. Depletion of cystatins from conditioned media or through siRNA-mediated silencing reduced the ability of ERβ to elicit these anti-tumor effects. In vivo, estrogen treatment of mice harboring ERβ+ TNBC cell line xenografts or PDXs resulted in increased tumoral expression and serum levels of cystatins, and suppressed tumor growth.

Conclusions: Our data demonstrated that estrogen and ERβ-specific agonists elicit anti-cancer effects in ERβ+ TNBC, both in vitro and in vivo. These effects are partially mediated by cystatins which can interact with, and inhibit, canonical TGFβ signaling, a pathway known to drive TNBC progression. Given the lack of targeted therapies for TNBC patients, the present data suggests that estrogen or ERβ-specific agonists offer a novel approach to manage this subset of patients.

Citation Format: Reese JM, Bruinsma ES, Suman VJ, Nelson AW, Chernukhin I, Carroll JS, Ingle JN, Goetz MP, Hawse JR. Biological functions of ERβ in triple negative breast cancer and its utility as a novel therapeutic drug target [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-07-20.

Abstract PD2-03: Final results of a first-in-human phase I study of the tamoxifen (TAM) metabolite, Z-Endoxifen hydrochloride (Z-Endx) in women with aromatase inhibitor (AI) refractory metastatic breast cancer (MBC) (NCT01327781)

Background: AI's are more effective than TAM in ER+ breast cancer. In AI refractory MBC, the response rate to TAM is 0% (Osborne 2011). Z-Endx is an active metabolite of TAM and among TAM treated women in the adjuvant and metastatic settings, reduced CYP2D6 metabolism and low Endx concentrations (Css <20 nM) have been associated with increased likelihood of disease recurrence. Preclinical studies have demonstrated greater Z-Endx exposure and anti-tumor activity with oral Z-Endx compared to equivalent doses of oral TAM (Reid 2014)

Methods: We conducted a phase I trial to determine the maximum-tolerated dose (MTD) and evaluate the toxicities, clinical activity, and pharmacokinetics (PK) of Z-Endx in patients (pts) with ER+, AI refractory MBC. Unlimited prior endocrine regimens were allowed. An accelerated titration schedule was applied (2 pts/dose level) until moderate toxicity or DLT, followed by a 3+3 design and then to expansion cohorts (40, 80, and 100 mg/day). Z-Endx was administered orally once daily (28 day cycle). Eye exams were performed at baseline, and end of cycles 2 and 6. PK was performed during cycle 1 and prior to subsequent cycles. For pts in the expansion cohorts, tumor biopsies were obtained at baseline for DNA sequencing (Foundation Medicine). Plasma cholesterol levels were obtained at baseline and after 1 cycle.

Results: From March 2011 to Dec 2014, 41 pts (38 evaluable), median age 60, received Z-Endx once daily encompassing 7 dose levels (20-160 mg/daily). The median number of prior hormonal regimens was 2 and 3 for the dose escalation and expansion cohorts, respectively. Dose escalation was stopped at 160 mg/day given MTD not reached and attainment of mean Endx Css of 3.6 uM. Cycle 1 DLT (PE) was observed in one patient (60 mg). No eye toxicity was observed. PK demonstrated mean Endx Css of > 1 uM at all dose levels ≥ 40 mg/day. Antitumor activity was observed at multiple dose levels including 3 confirmed partial responses and an additional 7 with stable disease for ≥6 cycles. Of these 10 pts, 9 had prior progression on both AI and fulvestrant and 3 additionally on TAM. After 1 cycle, total and LDL cholesterol decreased > 20 points in 54% and 40% of pts, respectively. Tumor sequencing in the expansion cohorts (n=14) did not identify ESR1 mutations; however, ESR1 amplification was identified in 1 pt with prolonged stable disease (>200 days). Of 6 pts with rapid progression (≤2 cycles), 4/6 had either CCND1 amplification (n=1) or at least one of the following activating mutations: ERBB2 L755S (n=1), AKT1 E17K (n=1), MTOR E1799K (n=1).

Conclusions: The direct administration of Z-END provides substantial drug exposure, acceptable toxicity, and "proof of principle" antitumor activity in endocrine resistant MBC. While the MTD was not determined, the goal of achieving Endx Css concentrations of > 1 uM was achieved. Tumor sequencing identified pts with predicted and confirmed endocrine resistance. A randomized phase II comparing endoxifen (80 mg/day) with TAM in AI refractory MBC was recently activated (NCT02311933). Supported in part by CA 133049, CA186686, CA15083, CA116201, and CA15083.

Citation Format: Goetz MP, Suman VJ, Reid JM, Northfelt DW, Mahr MA, Dockter T, Kuffel M, Buhrow SA, Safgren SL, McGovern RM, Collins JM, Streicher H, Hawse JR, Haddad TC, Erlichman C, Ames MM, Ingle JN. Final results of a first-in-human phase I study of the tamoxifen (TAM) metabolite, Z-Endoxifen hydrochloride (Z-Endx) in women with aromatase inhibitor (AI) refractory metastatic breast cancer (MBC) (NCT01327781). [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD2-03.

Abstract P6-04-03: ERb and breast cancer: A potential predictive and prognostic biomarker and novel therapeutic drug target

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 P5-05-01: Endoxifen, a novel breast cancer therapy, elicits unique beneficial effects on bone relative to that of other selective estrogen receptor modulators

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.

P3-16-09: Endoxifen, a Newly Developed Breast Cancer Drug, Has Anabolic Actions on the Mouse Skeleton

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 have recently demonstrated that endoxifen is the most active tamoxifen metabolite with regard to inhibiting the growth of ERα+ breast cancer cells and these studies have led to the development of endoxifen as a novel anti-breast cancer drug for which first-in-human studies are now underway. At present, there are no data regarding endoxifen's effects on bone.

Methods: The effects of endoxifen on osteoblast (OB) and osteoclast (OC) maturation and gene expression were monitored by cell differentiation assays and real-time PCR. Dual-energy X-ray absorptiometry (DXA), peripheral Quantitative Computed Tomography (pQCT) and micro-Computed Tomography (μCT) were used to determine changes in bone density, mass and architecture following 45 days of oral endoxifen administration (50mg/kg/day) to 3-month-old ovariectomized (OVX) C57BL/6 mice relative to vehicle control treated animals. Alterations in the numbers and activity of OBs and OCs were determined by histomorphometry and serum levels of P1NP and CTX-1 respectively.

Results: Endoxifen treatment of mouse derived bone marrow stromal cells and human OBs led to significant increases in the expression of critical bone marker genes such as Runx2, osterix, osteocalcin, osteoprotegerin and alkaline phosphatase in a dose dependent manner. Daily administration of endoxifen to OVX mice led to significant increases in total body bone mineral density (BMD) (6%) and content (BMC) (9%), which was accompanied by a 50% decrease in fat tissue mass as determined by DXA. pQCT analysis of the tibial metaphysis revealed dramatic increases in BMD (35%) and BMC (20%), as well as trabecular density (52%), cortical content (62%), cortical area (60%) and cortical thickness (78%). μCT analysis of the femoral metaphysis revealed increases in bone volume/total volume (200%), trabecular number (38%) and trabecular thickness (18%), as well as decreased trabecular spacing (29%). Interestingly, there was nearly a 50% increase in the numbers of OCs derived from endoxifen treated mice which was associated with elevated expression of OC marker genes such as NFATcl, RANK, c-fms and cathepsin-K compared to control treated animals. Approximately 4 times as many OBs and OCs were observed on the bone surfaces of endoxifen treated mice which correlated with nearly 2-fold increases in serum levels of the bone formation (P1NP) and resorption (CTX-1) markers.

Conclusions: These data are the first to demonstrate that endoxifen has anabolic effects on the mouse skeleton which are similar to that of estrogen. Additionally, these data reveal that endoxifen's mechanism of action in bone is different than that reported for tamoxifen and other selective estrogen receptor modulators in mice as it increases, rather than decreases, bone formation and remodeling. Therefore, the use of endoxifen for the treatment of endocrine responsive breast cancer may avoid the detrimental skeletal effects of many conventional endocrine therapies.

Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-16-09.

P4-02-03: Biological Functions of Estrogen Receptor-beta and Its Variants in Breast Cancer

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.

TGFbeta inducible early gene-1 directly binds to, and represses, the OPG promoter in osteoblasts

TGFbeta inducible early gene-1 (TIEG) is a member of the Krüppel-like family of transcription factors (KLF10) that plays an important role in TGFbeta mediated Smad signaling. In order to better understand the role of TIEG in bone, we generated TIEG knockout (KO) mice. Calvarial osteoblasts (OBs) isolated from these mice exhibit a reduced ability to support osteoclastogenesis in vitro. Gene expression studies revealed decreased receptor activator of NF-kappaB ligand (RANKL) and increased osteoprotegerin (OPG) expression in TIEG KO OBs, suggesting a potential role for TIEG in regulating the expression of these genes. Since OPG and RANKL are two important regulators of osteoclast (OC) differentiation, we sought to determine if TIEG directly regulates their expression. Luciferase constructs, containing fragments of either the mouse OPG promoter (-1486 to +133 bp) or the RANKL promoter (-2000 to +1 bp) were each cloned into the pGL3 basic reporter vector and transiently transfected into TIEG KO calvarial OBs with and without a TIEG expression vector. No significant changes in the activity of the RANKL promoter were detected in the presence of TIEG. However, OPG promoter activity was inhibited in the presence of TIEG protein suggesting that TIEG directly represses the expression of OPG in OBs. In order to determine the region of this promoter through which TIEG acts, sequential 5'-deletion constructs were generated. Transient transfection of these constructs revealed that the TIEG regulatory element(s) reside within a 200 bp region of the OPG promoter. Transient ChIP analyses, using a TIEG-specific antibody, revealed that TIEG binds to this region of the OPG promoter. Since we have previously shown that TIEG regulates target gene expression through Sp-1 sites, we examined this region of the OPG promoter for potential TIEG binding elements and identified four potential Sp-1 binding sites. Site-directed mutagenesis was used to determine if TIEG utilizes these Sp-1 elements to regulate the activity of the OPG promoter. The data demonstrate that two Sp-1 sites are likely to be involved in TIEG's repression of the OPG promoter. Taken together, these results confirm that TIEG directly binds to and inhibits OPG promoter activity in OBs, partially explaining the inability of TIEG KO OBs to fully support osteoclast differentiation.

Endoxifen, but not 4-hydroxytamoxifen, degrades the estrogen receptor in breast cancer cells: a differential mechanism of action potentially explaining CYP2D6 effect

Abstract #19

Background: Tamoxifen (TAM) is a standard endocrine therapy for the treatment of women with estrogen receptor (ER) positive breast cancer. TAM is activated by the cytochrome P450 2D6 enzyme system into two potent and active metabolites, 4-hydroxytamoxifen (4HT) and 4-hydroxy-N-desmethyl-tamoxifen (endoxifen). While human concentrations of 4HT are negligible and vary little in plasma (5-10 nM), endoxifen concentrations vary widely (10-180 nm), and women with genetically impaired CYP2D6 metabolism have significantly reduced endoxifen levels and a higher risk of breast cancer recurrence. Despite these observations, endoxifen's contribution to tamoxifen's overall drug effectiveness is uncertain.
 Methods: Using cells endogenously expressing ERa (MCF7, T47D) and cells stably transfected with ERa (Hs578T and U2OS), we examined the relative effects of TAM and its primary metabolites on ERa protein levels by western blotting, ERa transcriptional activity by luciferase reporter assays and real-time RT-PCR, and on ER positive breast cancer cell growth through the use of proliferation assays.
 Results: We have discovered that endoxifen induces ERa protein turnover through proteasomal degradation similar to that of ICI in a concentration and time-dependent manner. These findings are in stark contrast to TAM, N-desmethyl-tamoxifen (NDT) and 4HT, which stabilize the ER. Optimal degradation occurs only at endoxifen concentrations observed in human CYP2D6 extensive metabolizers (> 40 nM) and persists even in the presence of TAM (300 nM), 4HT (7 nM), and NDT (700 nM) at concentrations observed in patients receiving tamoxifen therapy. In contrast, reducing endoxifen concentrations to those observed in a CYP2D6 poor metabolizer (20 nM), without altering TAM, 4HT, and NDT, results in ER stabilization. High endoxifen concentrations (100-1000 nM) completely block estrogen (E2)-induced ER transcriptional activity even in the presence of TAM, 4HT, and NDT, while low endoxifen concentrations (20-40 nM) do not. Further, low concentrations of endoxifen (20 nM) do not significantly alter E2-induced cell proliferation; however, high concentrations of endoxifen (100-1000 nM) completely block this process. Discussion: Our data demonstrate that endoxifen is a potent anti-estrogen that targets ERa for proteasomal degradation, blocks ERa transcriptional activity and inhibits E2-induced breast cancer cell proliferation. Importantly, these effects of endoxifen are observed at concentrations found in CYP2D6 extensive metabolizers and are maintained even in the presence of TAM, 4HT and NDT. These studies suggest that endoxifen may be the primary metabolite responsible for tamoxifen's effectiveness in the treatment of ER positive breast cancer and provide the impetus to evaluate whether TAM-related side-effects, such as thrombo-embolism and endometrial hyperplasia/carcinoma, are inversely associated with a patient's ability to metabolically activate tamoxifen.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 19.

TIEG-null mice display an osteopenic gender-specific phenotype

TGFbeta inducible early gene-1 (TIEG) was originally cloned from human osteoblasts (OB) and has been shown to play an important role in TGFbeta/Smad signaling, regulation of gene expression and OB growth and differentiation. To better understand the biological role of TIEG in the skeleton, we have generated congenic TIEG-null (TIEG(-/-)) mice in a pure C57BL/6 background. Through the use of DXA and pQCT analysis, we have demonstrated that the femurs and tibias of two-month-old female TIEG(-/-) mice display significant decreases in total bone mineral content, density, and area relative to wild-type (WT) littermates. However, no differences were observed for any of these bone parameters in male mice. Further characterization of the bone phenotype of female TIEG(-/-) mice involved mechanical 3-point bending tests, micro-CT, and histomorphometric analyses of bone. The 3-point bending tests revealed that the femurs of female TIEG(-/-) mice have reduced strength with increased flexibility compared to WT littermates. Micro-CT analysis of femurs of two-month-old female TIEG(-/-) mice revealed significant decreases in cortical bone parameters compared to WT littermates. Histomorphometric evaluation of the distal femur revealed that female TIEG(-/-) mice also display a 31% decrease in cancellous bone area, which is primarily due to a decrease in trabecular number. At the cellular level, female TIEG(-/-) mice exhibit a 42% reduction in bone formation rate which is almost entirely due to a reduction in double labeled perimeter. Differences in mineral apposition rate were not detected between WT and TIEG(-/-) mice. Taken together, these findings suggest that female TIEG(-/-) mice are osteopenic mainly due to a decrease in the total number of functional/mature OBs.

Identification and functional clustering of global gene expression differences between age-related cataract and clear human lenses and aged human lenses

We have examined the gene expression profiles of young, old and cataractous human lenses in order to differentiate those gene expression changes specific for cataract from those also associated with lens aging. Differentially expressed transcripts were identified by oligonucleotide microarray analysis and clustered according to their known functions. Four hundred and twelve transcripts that are increased and 919 transcripts that are decreased were identified at the 2-fold or greater level between epithelia isolated from cataract relative to clear lenses while 182 transcripts that are increased and 547 transcripts that are decreased were identified at the 2-fold or greater level between young and old lens epithelia. Comparison of the cataract gene expression changes with those detected in lens aging revealed that only 3 transcripts exhibited similar trends in gene expression. These data suggest that cataract- and age-specific changes in gene expression do not overlap and provide evidence for multiple cataract- and age-specific gene expression changes in the human lens.