Pharmacological induction of chromatin remodeling drives chemosensitization in triple-negative breast cancer

Targeted therapies have improved outcomes for certain cancer subtypes, but cytotoxic chemotherapy remains a mainstay for triple-negative breast cancer (TNBC). The epithelial-to-mesenchymal transition (EMT) is a developmental program co-opted by cancer cells that promotes metastasis and chemoresistance. There are no therapeutic strategies specifically targeting mesenchymal-like cancer cells. We report that the US Food and Drug Administration (FDA)-approved chemotherapeutic eribulin induces ZEB1-SWI/SNF-directed chromatin remodeling to reverse EMT that curtails the metastatic propensity of TNBC preclinical models. Eribulin induces mesenchymal-to-epithelial transition (MET) in primary TNBC in patients, but conventional chemotherapy does not. In the treatment-naive setting, but not after acquired resistance to other agents, eribulin sensitizes TNBC cells to subsequent treatment with other chemotherapeutics. These findings provide an epigenetic mechanism of action of eribulin, supporting its use early in the disease process for MET induction to prevent metastatic progression and chemoresistance. These findings warrant prospective clinical evaluation of the chemosensitizing effects of eribulin in the treatment-naive setting.

Pharmacological Induction of mesenchymal-epithelial transition chemosensitizes breast cancer cells and prevents metastatic progression

The epithelial-mesenchymal transition (EMT) is a developmental program co-opted by tumor cells that aids the initiation of the metastatic cascade. Tumor cells that undergo EMT are relatively chemoresistant, and there are currently no therapeutic avenues specifically targeting cells that have acquired mesenchymal traits. We show that treatment of mesenchymal-like triple-negative breast cancer (TNBC) cells with the microtubule-destabilizing chemotherapeutic eribulin, which is FDA-approved for the treatment of advanced breast cancer, leads to a mesenchymal-epithelial transition (MET). This MET is accompanied by loss of metastatic propensity and sensitization to subsequent treatment with other FDA-approved chemotherapeutics. We uncover a novel epigenetic mechanism of action that supports eribulin pretreatment as a path to MET induction that curtails metastatic progression and the evolution of therapy resistance.

Lineage plasticity enables low-ER luminal tumors to evolve and gain basal-like traits

Stratifying breast cancer into specific molecular or histologic subtypes aids in therapeutic decision-making and predicting outcomes; however, these subtypes may not be as distinct as previously thought. Patients with luminal-like, estrogen receptor (ER)-expressing tumors have better prognosis than patients with more aggressive, triple-negative or basal-like tumors. There is, however, a subset of luminal-like tumors that express lower levels of ER, which exhibit more basal-like features. We have found that breast tumors expressing lower levels of ER, traditionally considered to be luminal-like, represent a distinct subset of breast cancer characterized by the emergence of basal-like features. Lineage tracing of low-ER tumors in the MMTV-PyMT mouse mammary tumor model revealed that basal marker-expressing cells arose from normal luminal epithelial cells, suggesting that luminal-to-basal plasticity is responsible for the evolution and emergence of basal-like characteristics. This plasticity allows tumor cells to gain a new lumino-basal phenotype, thus leading to intratumoral lumino-basal heterogeneity. Single-cell RNA sequencing revealed SOX10 as a potential driver for this plasticity, which is known among breast tumors to be almost exclusively expressed in triple-negative breast cancer (TNBC) and was also found to be highly expressed in low-ER tumors. These findings suggest that basal-like tumors may result from the evolutionary progression of luminal tumors with low ER expression.

Phenotypic heterogeneity driven by plasticity of the intermediate EMT state governs disease progression and metastasis in breast cancer

The epithelial-to-mesenchymal transition (EMT) is frequently co-opted by cancer cells to enhance migratory and invasive cell traits. It is a key contributor to heterogeneity, chemoresistance, and metastasis in many carcinoma types, where the intermediate EMT state plays a critical tumor-initiating role. We isolate multiple distinct single-cell clones from the SUM149PT human breast cell line spanning the EMT spectrum having diverse migratory, tumor-initiating, and metastatic qualities, including three unique intermediates. Using a multiomics approach, we identify CBFβ as a key regulator of metastatic ability in the intermediate state. To quantify epithelial-mesenchymal heterogeneity within tumors, we develop an advanced multiplexed immunostaining approach using SUM149-derived orthotopic tumors and find that the EMT state and epithelial-mesenchymal heterogeneity are predictive of overall survival in a cohort of stage III breast cancer. Our model reveals previously unidentified insights into the complex EMT spectrum and its regulatory networks, as well as the contributions of epithelial-mesenchymal plasticity (EMP) in tumor heterogeneity in breast cancer.

Abstract 1602: Lineage plasticity enables low ER luminal tumors to evolve and gain basal-like traits

Stratifying breast cancer into specific molecular or histological subtypes aids in therapeutic decision-making and predicting outcomes, however, these subtypes may not be as distinct as previously thought. Patients with luminal-like, Estrogen Receptor (ER)-expressing tumors have better prognosis than patients with more aggressive, triple-negative or basal-like tumors. There is, however, a subset of luminal-like tumors that express lower levels of ER, which exhibit more basal-like features. Previous studies have suggested that triple negative, basal-like tumors may arise from a luminal cell-of-origin, but there are no definitive studies that identify the cell-of-origin of these low ER tumors.

Analysis of 2208 invasive breast carcinomas from 2012-2020 revealed that 2% of tumors have low ER expression (less than 10% ER positive cells), which are mostly high-grade carcinomas and exhibit basal-like features. TCGA analysis revealed that tumors with lower ER expression (lowest quartile of ER expression) expressed higher basal signature genes as compared to tumors with higher levels of ER expression. This variation within the ER+ subtype and the emergence of basal-like characteristics within low ER tumors suggest that some luminal tumors may evolve into a more basal-like or triple-negative subtype.

The luminal mouse mammary tumor, MMTV-PyMT, was used to model low ER human tumors and, similar to the patient tumor samples, basal-like tumor cells were also found within these tumors. Lineage tracing using tissue-specific and inducible Cre recombinase-based labelling was performed to elucidate the lineage-of-origin of these basal-like cells, revealing that these basal-like cells were derived from normal luminal epithelial cells, not basal cells.

Our study uncovers the existence of luminal-basal plasticity within tumors of a low ER subtype that enables these cells to transition into a more basal-like state. Understanding the factors that enable this plasticity to occur may reveal opportunities to curb the evolution of more aggressive traits, potentially improving the way breast cancer is currently managed and treated.

Citation Format: Gadisti Aisha Nurulhijjah Binti Mohamed, Nevena B. Ognjenovic, Sundis Mahmood, Sarah Min Kyung Lee, Brock C. Christensen, Kristen E. Muller, Diwakar R. Pattabiraman. Lineage plasticity enables low ER luminal tumors to evolve and gain basal-like traits [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 1602.

Abstract P3-06-01: Eribulin alters the chromatin landscape to induce MET, attenuating metastatic progression and sensitizing breast tumors to subsequent chemotherapy

Introduction: Two major hurdles faced in clinical cancer therapy are metastatic progression that results in the majority of cancer deaths, and resistance to therapy that fuels tumor relapse. The epithelial-mesenchymal transition (EMT) is a reversible cellular program that contributes to the intratumoral heterogeneity of carcinoma cells and confers traits that promote metastatic progression and resistance to chemotherapy. The standard of care for triple negative breast cancer (TNBC) remains neoadjuvant chemotherapy where cyclophosphamide, doxorubicin and paclitaxel are administered in combination. In case of advanced/metastatic disease, third-line eribulin is administered as a single agent. In some cases of advanced disease, vinorelbine has also demonstrated efficacy. Eribulin, vinorelbine, and paclitaxel are microtubule inhibitors that act via mitotic blockade. Knowledge gap: Despite major advances in our understanding, the contributions of EMT research to improvements in cancer therapy have been minimal and yielded no clinically viable strategies to selectively modulate EMT in human tumors. Results: Our new data reveals that eribulin, in addition to blocking tubulin polymerization, utilizes microtubule-independent mechanism to epigenetically induce MET. Using thermal proteome profiling and mass spectrometry analyses, we identify candidate target proteins of eribulin that enable it to act in an epigenetic fashion to induce MET.Treating TNBC patient-derived xenograft tumors with eribulin induces a significant reduction in metastatic burden, with the remaining metastatic colonies exhibiting a well-differentiated histopathology. Eribulin efficiently induce MET in cells that have received no prior chemotherapy; in cells pretreated with anthracyclines and taxanes, eribulin is less efficient at MET induction. Additionally, cells that were resistant to first-line taxane treatment were also resistant to eribulin-induced cell death. Using patient-derived xenograft (PDX) models, we have determined the optimal sequence of eribulin administration to maximize its MET-inducing and cytotoxic effects. Conclusions: We uncover that, working through altering the chromatin landscape and transcriptional profile of cells, eribulin induces MET and primes cells for subsequent chemotherapy. Additionally, the efficacy of eribulin is highest when administered in the treatment-naïve setting. By developing an understanding of eribulin’s mechanism of action, treatment strategies can be optimized to maximize therapeutic potential by exploiting its effects in both mitotic blockade as well as MET. Upon completion of this study, we will better understand how MET-induced tumor differentiation would work and the impact that it would have on current conventional therapeutic strategies. Acknowledgements: Eribulin studies funded by Eisai Inc.

Citation Format: Diwakar R Pattabiraman, Meisam Bagheri, Nevena B Ognjenovic, Ian S LaCroix, Scott A Gerber. Eribulin alters the chromatin landscape to induce MET, attenuating metastatic progression and sensitizing breast tumors to subsequent chemotherapy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-06-01.

Limiting Self-Renewal of the Basal Compartment by PKA Activation Induces Differentiation and Alters the Evolution of Mammary Tumors

Differentiation therapy utilizes our understanding of the hierarchy of cellular systems to pharmacologically induce a shift toward terminal commitment. While this approach has been a paradigm in treating certain hematological malignancies, efforts to translate this success to solid tumors have met with limited success. Mammary-specific activation of PKA in mouse models leads to aberrant differentiation and diminished self-renewing potential of the basal compartment, which harbors mammary repopulating cells. PKA activation results in tumors that are more benign, exhibiting reduced metastatic propensity, loss of tumor-initiating potential, and increased sensitivity to chemotherapy. Analysis of tumor histopathology revealed features of overt differentiation with papillary characteristics. Longitudinal single-cell profiling at the hyperplasia and tumor stages uncovered an altered path of tumor evolution whereby PKA curtails the emergence of aggressive subpopulations. Acting through the repression of SOX4, PKA activation promotes tumor differentiation and represents a possible adjuvant to chemotherapy for certain breast cancers.

Abstract 6430: Redefining therapy regimens for triple negative breast cancer - Exploiting the epigenetic effects of eribulin action

While the advent of targeted therapy has led to vast improvements in outcomes for certain types of breast cancer, the standard of care for triple negative breast cancer (TNBC) still remains chemotherapy. A major clinical hurdle in the successful management of this disease is the eventual development of therapeutic resistance and disease relapse in more aggressive forms. Our research centers on the improving therapeutic outcomes for TNBC through the induction of tumor differentiation. Current therapy regimens for TNBC involve the administration of anthracyclines such as doxorubicin, and taxanes such as paclitaxel, typically in combination. When patients present with advanced or metastatic disease, or upon relapse following treatment with an anthracycline or a taxane, eribulin is administered as third-line therapy. We observe that treatment of TNBC cell lines such as SUM159, BT549 or Hs578T, or mouse models such as MMTV-PyMT or C3(1)-Tag with either doxorubicin, paclitaxel or eribulin results in the emergence of resistant subpopulations of cells. In the case of doxorubicin and paclitaxel, the resistant clones exhibit mesenchymal properties due to therapy-induced epithelial-to-mesenchymal transition (EMT). In the case of eribulin, however, these resistant subpopulations are epithelial in nature, having undergone mesenchymal-to-epithelial transition (MET). Our work sheds like on the epigenetic mechanisms that drive eribulin-induced changes to cell state, providing insights into its non-antimitotic actions. Assaying the chromatin landscape of eribulin-resistant vs. paclitaxel-resistant cells by ATAC-Seq reveals genome-wide repression of transcription upon treatment with eribulin. This is accompanied by a severe loss in H3K4me3 that suppresses the transcription of thousands of promoter regions. By carrying out thermal proteome profiling, we identify targets of eribulin that could be responsible for its effects in epigenomic reprogramming and induction of differentiation. Surprisingly, doxorubicin- or paclitaxel-resistant cells are no longer susceptible to subsequent treatment with eribulin, do not undergo MET, and remain difficult to eliminate. Eribulin-resistant cells, however, owing to their epithelial phenotype, are eliminated by subsequent treatment with other drugs including doxorubicin and paclitaxel. Current treatment regimens in the clinic administer eribulin only to patients that have previously received anthracyclines and/or taxanes, never in the treatment-naive or neoadjuvant setting. Our findings highlight the importance of designing treatment regimens based on a clear understanding of the effects of individual drugs on cellular phenotype, including altered responsiveness induced by prior lines of therapy. Our work has major implications for the clinical use and the sequence of administration of chemotherapy with the potential to rapidly alter the course of tumor progression in TNBC by altering treatment regimens.

Citation Format: Meisam Bagheri, Aisha Mohamed, Nevena B. Ognjenovic, Diwakar R. Pattabiraman. Redefining therapy regimens for triple negative breast cancer - Exploiting the epigenetic effects of eribulin action [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6430.