The role of metabolic rewiring in endocrine resistance

Breast cancer is the most common cancer type in women population affecting a large number of women worldwide. Due to its highly heterogenous molecular characteristics, there is not a default treatment strategy for breast cancer. Almost two third of breast cancer cases are estrogen receptor positive ER (+). Majority of breast cancer specific deaths in women with ERα (+) tumor occur due to metastases that are resistant to endocrine therapy. There is a critical need for novel therapeutic approaches to prevent or delay recurrence of ERα (+) tumors. The overall objective of this dissertation project is to define the interaction among endocrine resistance, nuclear export system, and cellular metabolic pathways, and to evaluate the metabolic impact of combinational ER and XPO1 targeting by using combination of endocrine therapy agents and a XPO1-specific small molecule inhibitor (Selinexor-SEL) in different endocrine resistant ER (+) breast cancer cell lines. The long term goal is to develop effective treatment strategies to overcome endocrine resistance in ER (+) breast cancer. In the first part of this project, we focused on the characterization of XPO1 protein in tamoxifen resistance. In this part, we used endocrine-sensitive and -resistant breast cancer cell lines (in vitro) and tumor xenograft models (in vivo) to test our hypothesis. Our results showed that expression profile of XPO1 protein has a pivotal role in the progression of tamoxifen resistance in ER (+) breast cancer cell models. We found out that XPO1 protein modulates tamoxifen resistance due to its function to determine subcellular localization of important kinases. In the second part of the project, we investigated how nuclear export system contribute to tamoxifen resistance development by testing with a novel combinational targeting strategy using 4-OHT and SEL in Luminal B type breast cancer cell lines, and to investigate the metabolic outcomes of this novel therapy. To test our hypotheses, we used actual patient tumor samples and endocrine-sensitive and -resistant breast cancer cell lines (in vitro). Our findings indicated that XPO1 expression is significantly higher in Luminal B subtype compared to other molecular subtypes. We demonstrated that combined targeting of XPO1 and ERα rewires metabolic pathways (e.g. Akt pathway), and shuts down both glycolytic and mitochondrial pathways that would eventually lead to autophagy. Lastly, we assessed the impact of other endocrine therapy options alone or in combination with Selinexor on metabolism in different endocrine resistant breast cancer cell lines and possible metastatic organ sites. We used 3-D cell culture models and endocrine-sensitive and -resistant breast cancer cell lines (in vitro). Using a combination of transcriptomics, kinase arrays, metabolomics and metabolic flux experiments, we identified glutamine metabolism pathways to be rewired during endocrine resistance. In limited media conditions mimicking nutrient deprived tumor microenvironment, endocrine resistant cells were more dependent on mitochondria for energy production. Their glucose and fatty acid dependency decreased in the presence of SEL and cells were more dependent on glutamine. The effect of glutamine was dependent on conversion of the glutamine to glutamate and mitochondrial complex 1 activity. In order to examine metabolites that might result in the observed phenotype we performed GC/MS whole metabolite profiling and identified amino acid metabolism pathways to be upregulated when cells were treated with SEL. In conclusion, our study indicated that remodeling metabolic pathways to regenerate new vulnerabilities in endocrine resistant breast tumors is novel, and given the need for better strategies for improving therapy response of relapsed ERα(+) tumors, our findings show great promise for uncovering the role ERα-XPO1 crosstalk plays in reducing cancer recurrences