Endocrine Therapy Resistant Cells Research Articles

Targeting aberrant fatty acid synthesis and storage in endocrine resistant breast cancer cells.

Lipid metabolic reprogramming is an emerging characteristic of endocrine therapy (ET) resistance in estrogen receptor-positive (ER+) breast cancer. We explored changes in lipid metabolism in ER+ breast cancer cell lines following acquired resistance to common endocrine treatments and tested efficacy of an inhibitor in current clinical trials. We derived ER+ breast cancer cell lines resistant to Tamoxifen (TamR), Fulvestrant (FulvR), and long-term estrogen withdrawal (EWD). Parental and ET resistant cells were subjected to global gene expression and unbiased lipidomic profiling. Lipid storage changes were assessed via neutral lipid staining with Oil Red O (ORO). The impact of the fatty acid synthase (FASN) inhibitor TVB-2640 on the growth and lipid storage of these cell lines was evaluated. Additionally, 13 C 2 -acetate tracing was used to examine FASN activity in parental and ET resistant cells in the absence or presence of TVB-2640. Compared to parental cells, lipid metabolism and processing pathways were notably enriched in ET resistant cells, which exhibited distinct lipidomes characterized by increased triglyceride and polyunsaturated FA (PUFA) species. ET-resistant cells displayed enhanced cytoplasmic lipid droplets. Increased FASN protein levels were observed in ET-resistant cells, and TVB-2640 effectively inhibited FASN activity. FASN inhibition reduced cell growth in some but not all cell lines and ET resistance types and did not correlate to lipid storage reduction. 13 C 2 -acetate tracing confirmed reduced palmitate synthesis and enhanced PUFA synthesis in ET-resistant cells, especially when combined with FulvR. ET resistant breast cancer cells exhibit a shift towards enhanced triglyceride storage and complex lipids enriched with PUFA acyl chains. While targeting FASN alongside ET may not fully overcome ET resistance in our models, focusing on the unique lipid metabolic dependencies, such as PUFA pathways, may present a promising alternative strategy for treating ET resistant breast cancer.

Exosc9 Initiates SUMO-Dependent lncRNA TERRA Degradation to Impact Telomeric Integrity in Endocrine Therapy Insensitive Hormone Receptor-Positive Breast Cancer.

Long, noncoding RNAs (lncRNAs) are indispensable for normal cell physiology and, consequently, are tightly regulated in human cells. Yet, unlike mRNA, substantially less is known about the mechanisms for lncRNA degradation. It is important to delineate the regulatory control of lncRNA degradation, particularly for lncRNA telomeric repeat-containing RNA (TERRA), as the TERRA-telomere R-loops dictate cell cycle progression and genomic stability. We now report that the exosome complex component Exosc9 degrades lncRNA TERRA in human mammary epithelial cells. Heterochromatin protein 1 alpha (HP1α) recruits Exosc9 to the telomeres; specifically, the SUMO-modified form of HP1α supports interaction with Exosc9 and, as previously reported, lncRNA TERRA. The telomeric enrichment of Exosc9 is cell cycle-dependent and consistent with the loss of telomeric TERRA in the S/G2 phase. Elevated Exosc9 is frequently observed and drives the growth of endocrine therapy-resistant (ET-R) HR+ breast cancer (BCa) cells. Specifically, the knockdown of Exosc9 inversely impacts telomeric R-loops and the integrity of the chromosome ends of ET-R cells. Consistently, Exosc9 levels dictate DNA damage and the sensitivity of ET-R BCa cells to PARP inhibitors. In this regard, Exosc9 may serve as a promising biomarker for predicting the response to PARP inhibitors as a targeted monotherapy for ET-R HR+ BCa.

Abstract 2307: Sequential treatment with abemaciclib plus endocrine therapy inhibits cell proliferation and triggers apoptosis in cell lines resistant to CDK4/6i

Abstract CDK4/6 inhibitors combined with endocrine therapy (ET) have shown clinical benefit in HR+, HER2- breast cancer. However, development of resistance highlights the need for treatment strategies to improve outcomes. To study resistance mechanisms, breast cancer cell lines were treated with CDK4/6i (abemaciclib or palbociclib) in combination with 4-OH-tamoxifen (tamoxifen) for 120-144h and sorted for resistant cells defined as geminin positive (GEM+), a marker of S/G2/M cell cycle accumulation. To confirm the resistant phenotype, cell lines were treated with tamoxifen plus the CDK4/6 inhibitor used to drive resistance. To understand if sequential CDK4/6i treatment is effective in controlling cell proliferation, resistant cells generated through treatment with palbociclib + tamoxifen or abemaciclib + tamoxifen were treated with abemaciclib + ET (fulvestrant or tamoxifen) or palbociclib + ET, respectively. Geminin/Ki67, annexin V and colony formation assays were performed to evaluate cell proliferation and viability. Molecular characterization by western blot and RNAseq analysis were utilized to provide insights into mechanisms of resistance and the effects of sequential treatment with CDK4/6i + ET. Cell lines resistant to palbociclib + tamoxifen subsequently treated with abemaciclib + ET showed decreased %GEM+ and colony formation ability, decreased Ki67 levels, and increased apoptosis. These effects were not observed in cell lines resistant to abemaciclib + tamoxifen following subsequent treatment with palbociclib + ET. Western blot analysis showed that palbociclib and abemaciclib-resistant cells had increased CDK6 and pERK levels, compared to control. Importantly, treatment of palbociclib-resistant cells with abemaciclib + ET decreased FOXM1, a key regulator of senescence and apoptosis. Cyclin A, a marker of mitosis, was also decreased, consistent with decreased %GEM+ subpopulation in palbociclib + ET resistant cells. These effects were not observed in abemaciclib-resistant cells treated with palbociclib + ET.In summary, our in vitro data provides mechanistic insights into resistance to combination CDK4/6i + tamoxifen and suggests the potential benefit of sequential treatment with abemaciclib + ET to overcome resistance in breast cancer. Citation Format: Elisabet Zapatero-Solana, Maria P. Ganado, Maria J. Ortiz-Ruiz, Cecilia Mur, Lacey Litchfield, Farhana Merzoug, Oscar Puig, Maria Jsoe Lallena. Sequential treatment with abemaciclib plus endocrine therapy inhibits cell proliferation and triggers apoptosis in cell lines resistant to CDK4/6i [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2307.

Abstract 733: Significance of PELP1/SETDB1 axis in endocrine therapy resistance

Abstract BACKGROUND: PELP1 is commonly overexpressed in breast cancer (BCa) and is implicated in endocrine therapy resistance. However, the mechanism by which PELP1 contributes to therapy resistance remains elusive. Our ongoing studies using yeast two hybrid screen identified histone methyltransferase SETDB1 as a novel interactor of PELP1. The objective of this study is to determine the significance of PELP1 interaction with SETDB1 in BCa endocrine therapy resistance. METHODS: We used two ER+ BCa cell lines (MCF7 and ZR75) and three endocrine therapy resistant cell lines (Tamoxifen resistant MCF7-TamR, Fulvestrant resistant MCF7-FR, Tamoxifen resistant MCF7-PELP1-cyto). Additional BCa models with stable overexpression and under expression of SETDB1 and PELP1 (MCF7-SETDB1-PELP1shRNA, ZR75-SETDB1-PELP1shRNA, ZR75-SETDB1, MCF7-SETDB1, MCF7-TamR-SETDB1shRNA, MCF7-FR-SETDB1shRNA) were generated using lentiviral transduction. Functional significance of cross-talk was tested using MTT, soft agar, and colony formation assays. Mechanistic studies were conducted using immunoprecipitation, Western blotting, and RT-qPCR. RESULTS: Analyses of TCGA databases showed that SETDB1 expression is positively correlated with PELP1 expression in ER+ BCa (r=0.30, p<0.0001). Immunoprecipitation assays using multiple ER+ BCa cell lysates confirmed the interaction of SETDB1 with PELP1. Using ER+ BCa models with overexpression of SETDB1 or knockdown, we provided evidence that SETDB1 plays an important role in the proliferation of BCa cells. PELP1 knockdown attenuated SETDB1 mediated BCa cells proliferation. SETDB1 upregulation contributed to tamoxifen resistance, while PELP1 knockdown re-sensitized cells to tamoxifen therapy. Further, endocrine therapy resistant model cells (MCF7-TamR, MCF7-FR, MCF7-PELP1-cyto) showed increased expression of SETDB1, and its knockdown sensitized them to endocrine therapy. Mechanistic studies revealed that PELP1 is needed for SETDB1 mediated Akt phosphorylation and activation of its downstream targets such as S6, and Cyclin D1. CONCLUSIONS: The results of study suggest that the PELP1/SETDB1 interactions contribute to endocrine therapy resistance via aberrant activation of Akt signaling. Targeting PELP1/SETDB1 axis could represent a new therapeutic avenue to mitigate endocrine resistance. Supported by VA grant I01BX004545 (R.K. Vadlamudi) Citation Format: Zexuan Liu, Junhao Liu, Weiwei Tang, Uday P. Pratap, Kristin A. Altwegg, Behnam Ebrahimi, Xiaonan Li, Gangadhara R. Sareddy, Suryavathi Viswanadhapalli, Ratna K. Vadlamudi. Significance of PELP1/SETDB1 axis in endocrine therapy resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 733.

Abstract P6-04-26: PI3K inhibitors restored the sensitivity to anti-estrogen drugs in endocrine therapy resistant cell lines

Abstract Background The Combination of endocrine therapy with molecular targeted drugs has become one of the standard treatment option for ER positive advanced or metastatic breast cancer. The therapeutic effects of PI3K-mTOR inhibitors, which inhibit intracellular phosphorylation of signaling pathways, have been clarified in clinical trials. However, the effects of these drugs on ER signal pathways in endocrine-resistant breast cancer have not been fully elucidated. We investigated the changes in the expression and functions of ER when endocrine-resistant cell lines, established in our laboratory, were treated with PI3K-mTOR inhibitors Methods Long-term estrogen deprivation-resistant (EDR) cell lines and fulvestrant-resistant cell lines (MFR) were established from MCF-7 cells in our previous studies. These cell lines exhibited different ER expression levels, including high (EDR-1), low (EDR-2), or no expression (MFR). All these cell lines had the same PIK3CA mutations as the parental cell lines. We used buparlisib (BUP, pan-class1 PI3K inhibitor) and alpelisib (ALP, α-specific PI3K inhibitor) as PI3K inhibitors and everolimus (EVE) as an mTOR inhibitor. Tamoxifen and fulvestrant were used in anti-estrogen therapy. In this study, the procedure to examine histone modifications was CHIP-qtPCR analysis. Results EDR-1, -2 and MFR were treated with the PI3K-TOR inhibitors for 24 h. There were no changes in ER expression and ER activity on EDR-1 and MFR before and after the addition of PI3K-mTOR inhibitors. Interestingly, ER expression and ER activity in EDR-2 increased after the addition of PI3K-mTOR inhibitors. In particular, a high re-expression of ER was observed when the PI3K inhibitors were added. Next, in order to evaluate the effects of endocrine therapy on these cell lines Using BUP, ALP and EVE, each drug resistant cell lines were established from EDR-2. When the cell proliferation assay was performed in the established resistant cell line, EDR-2 which showed re-expression of ER on adding the PI3K inhibitors, showed cell growth-inhibitory effects with the addition of anti-estrogen agents. Previously, we reported that epigenetic regulation contributes to the reduction of ER expression in EDR-2. (Tsuboi et al, J Steroid Biochem Mol Biol 2017). Therefore, we investigated the possibility that epigenetic regulation could contribute to the mechanism of ER re-expression when PI3K inhibitors were used in EDR-2. It was revealed that the change of H3K27me3, one of the histonemodifications, correlated with the expression of ER. Conclusion PI3K inhibitor could restore the expression of ER and sensitivity to anti-estrogen drugs in endocrine therapy resistant cell line in which ER expression was lost during acquired resistant period. This restoration might be due to a histone modification (H3K27me3). Citation Format: Emi Tokuda, Shunta Sasaki, Kouki Tsuboi, Masafumi Iida, Toshifumi Niwa, Shigehira Saji, Shin-Ichi Hayashi. PI3K inhibitors restored the sensitivity to anti-estrogen drugs in endocrine therapy resistant cell lines [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-26.

Abstract 4896: The role of SLC7A5 (LAT1) in endocrine therapy-resistant breast cancer

Abstract Breast cancer is the leading cancer diagnosis for women in the United States. Endocrine therapies, such as tamoxifen and aromatase inhibitors, are used to treat the estrogen receptor-positive (ER+) breast cancers that comprise 70% of all new cases. Unfortunately, emergence of resistance to these therapies presents a major clinical challenge. Cancer cells can adapt to dysregulate the bioenergetics of cellular metabolism and evade cell death. Solute carrier family 7 member 5 (SLC7A5 or LAT1) is expressed across the cell membrane and is a transporter of large, neutral amino acids (such as leucine or tyrosine) supporting cell proliferation. Using LCC9 breast cancer cells that are resistant to tamoxifen and fulvestrant, we studied the role of LAT1 in endocrine therapy-resistant cells as compared to sensitive MCF7 breast cancer cells. SLC7A5 expression was upregulated by estrogen in MCF7 cells that was blocked in the presence of fulvestrant treatment. A significant 2.75-fold upregulation of SLC7A5 protein and 71-fold upregulation of SLC7A5 mRNA was found in LCC9 cells (as compared with MCF7 cells) without any estrogen regulation. Fulvestrant treatment did not significantly alter SLC7A5 mRNA or protein expression. These data suggest that higher levels of SLC7A5 in resistant cells may help in transporting key amino acids from their microenvironment to support cell growth. Inhibiting the functions of SLC7A5 using a pharmacologic inhibitor, JPH203, or depleting its expression by using siRNA led to significant growth suppression of LCC9 cells. Cell cycle analysis revealed that SLC7A5 depletion increased G1 phase of cell cycle and reduced S phase cells. In five publicly available data sets, of estrogen receptor-positive and tamoxifen-treated breast cancer patients, high expression of SLC7A5 was associated with worse prognosis. Endocrine therapy resistance is partly driven by autophagy in breast cancer cells. We depleted SLC7A5 expression in LCC9s to better understand the role of this transporter in autophagy. This study uncovers a novel adaptive mechanism in endocrine therapy-resistant breast cancer cells that is facilitated by increased SLC7A5 mediated transport of large neutral amino acids enabling them to supplement their increased metabolic needs and promoting cell growth. Therefore, blocking the functions of SLC7A5, perhaps in conjunction with inhibition of autophagy, may offer an avenue of potential therapeutic intervention in endocrine therapy-resistant breast cancers. Citation Format: Catherine M. Sevigny, Surojeet Sengupta, Zhexun Luo, Lu Jin, Dominic Pearce, Robert Clarke. The role of SLC7A5 (LAT1) in endocrine therapy-resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4896.

Abstract P3-04-25: Role of H19, a long non-coding RNA, in development of resistance to endocrine therapy in breast cancer cells

Abstract Introduction: Majority of breast cancer tumors are Estrogen receptor positive (ER+) where antiestrogen therapies (endocrine therapies) are the best therapeutic strategy to treat this type of tumors. However, eventually over 30% of patients will develop resistance to endocrine therapies resulting in disease relapse. We recently showed that the long noncoding RNA, H19, is an estrogen target gene that plays a significant role in estrogen-induced proliferation of the normal and malignant ER+ cells. We therefore hypothesize that H19 expression is also important to the proliferation of endocrine therapy resistant cells. In this study, we examined if estrogen-independent H19 expression is important to the development of endocrine therapy resistance. Objective: The overall objective of this project is to use therapy sensitive (MCF-7) and therapy-resistant (LCC9) breast cancer cells as model systems to examine the role of long non-coding RNA H19 in development and maintenance of resistance to endocrine therapy. Methodology and Results: We examined the expression of H19 in ER+ breast cancer cells (MCF7) that under the selective pressure of fulvesterant (ICI, ER down regulator) acquire resistance to ICI. We observed that while H19 expression was initially decreased as expected, its expression subsequently increased in the ICI-resistant MCF7 cells. Interestingly, H19 knockdown in MCF7 cells significantly decrease their proliferation as determined by Flowcytometry and made them more sensitive to ICI. We also examined H19 expression in the ICI-resistant LCC9 cells and found that ICI treatment increased H19 expression. Interestingly, H19 knockdown in the LCC9 cells decreased their proliferation and surprisingly made them sensitive to ICI treatment. Previous observations indicate that NOTCH4 receptor (NR4) may be involved in endocrine therapy resistance. Interestingly we found that in presence of ICI, NR4 expression is increased and that forced activation of NR4 markedly increases H19 expression in LCC9 cells. Conclusion: Altogether these observations suggest that H19 plays an important role in the development of endocrine therapy resistance and further our understanding of the cellular and molecular mechanisms involved in endocrine therapy resistance. These and similar studies could potentially lead to the development of new therapies to treat therapy resistant tumor cells. Further experiments would reveal if signalling pathways that regulate H19 expression independent of estrogen are useful therapies against endocrine therapy resistant tumors. Citation Format: Basak P, Chatterjee S, Bhat V, Jin H, Su A, Murphy LC, Raouf A. Role of H19, a long non-coding RNA, in development of resistance to endocrine therapy in breast cancer cells [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 P3-04-25.

Endocrine therapy resistance and epithelial to mesenchymal transition are driven by Nicastrin and Notch4 cooperation in MCF7 breast cancer cells

Endocrine therapy resistant (ETR) tumors often display mesenchymal features, associated with aggressive and enhanced motility behaviour. Notch signalling is over-activated in ETR cells. By blocking it, it is possible to interfere with the cell growth. Notch is also implicated in regulating epithelial to mesenchymal transition (EMT) affecting both migration and invasion in breast cancer cells. We know that Nicastrin is the major component of the multi-subunit protease complex Gamma Secretase (GS) which executes intramembrane proteolysis of integral proteins such Notch. We used two models of Endocrine Therapy Resistant MCF7 breast cancer cell lines which acquired EMT feature and showed higher levels of Nicastrin and Notch targets. Moreover, they displayed higher Notch4 levels but lower Notch1 and Notch2, indicating a possible switch of the Notch signalling when cells acquire resistance. We demonstrated that Gamma Secretase inhibitor PF03084014 and anti-Nicastrin MAbs were particularly effective in partially inhibiting the EMT process by blocking Nicastrin-Notch4 contribution to resistance. Importantly, we also demonstrated that the stem cells-like population was reduced upon these treatments. Both Nicastrin and Notch4 genetic silencing lead to similar effects. Finally, stably overexpressing Nicastrin, was sufficient to activate Notch4 in MCF7 and render them unresponsive to tamoxifen. Here we highlight our recent study.

Abstract 4995: Glucose-regulated protein 78 regulates crosstalk between apoptosis and autophagy to determine antiestrogen responsiveness

Abstract While over 70% of breast cancers express estrogen receptor α (ER+), drugs targeting these receptors such as Tamoxifen (TAM) or Faslodex (fulvestrant, ICI) often fail to cure these patients. Many ER+ patients fail to respond (de novo resistance) or lose drug responsiveness (acquired resistance), making endocrine resistance a major clinically relevant problem. The molecular mechanisms of endocrine therapy resistance still remain unclear. We show that antiestrogen resistant cells with an acquired resistant phenotype overexpress glucose-regulated protein 78 (GRP78), which is not observed in de novo resistance. Moreover, we show increased expression of GRP78 in MCF7/LCC9 (estrogen independent, ICI resistant, TAM cross-resistant) when compared to the MCF7/LCC1 (estrogen independent, TAM and ICI sensitive) breast cancer cells and tumors. These data reflect the response of aromatase inhibitor resistant breast cancers to second line antiestrogen therapy, and further support the hypothesis that antiestrogen resistance and estrogen independence are separate phenotypes. Targeting GRP78 with RNAi restored antiestrogen sensitivity in endocrine therapy resistant cells (MCF7-RR and MCF7/LCC9), whereas expressing GRP78 cDNA conferred resistance on antiestrogen sensitive breast cancer cells (MCF7 and MCF7/LCC1). We show that GRP78 integrates cellular signaling to inhibit apoptosis (as determined by cleaved caspase-7, cleaved PARP, and annexin V staining) and stimulate prosurvival autophagy (as determined by LC3-II, p62, and GFP-LC3 punctate). Moreover, GRP78-stimulated autophagy is dependent on TSC2/AMPK-mediated mTOR inhibition. Endocrine resistance induced by GRP78 overexpression is prevented through an inhibition of autophagy by ATG5 knockdown, while caspase inhibition prevented GRP78 RNAi-mediated therapy resensitization. Furthermore, knockdown of GRP78 and beclin 1 (BECN1) synergistically restores antiestrogen sensitivity in resistant cells. The use of GRP78 to integrate the cellular functions of apoptosis and autophagy to control cell fate raises the provocative question that this signaling may be widely applicable and represent a major stress response that explains how many different cell types could respond to stressors and acquire resistance to therapeutic interventions. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4995. doi:1538-7445.AM2012-4995

ER and PI3K Independently Modulate Endocrine Resistance in ER-Positive Breast Cancer

Endocrine therapy-resistant estrogen receptor-positive (ER(+)) breast cancer is the most common cause of breast cancer death. Miller and colleagues demonstrate that ligand-independent ER activity promotes the growth of breast cancer cells through CDK4/E2F. As an independent event, the phosphatidylinositol 3-kinase (PI3K) pathway is also upregulated in endocrine therapy-resistant cells. Promising preclinical evidence by several groups for the combination of an inhibitor of ligand-independent ER, fulvestrant, with PI3K inhibition, has led to the activation of trials evaluating this concept.