Purpose: Mutations in the ligand-binding domain (LBD) of estrogen receptor a (ER) confer constitutive transcriptional activity and resistance to endocrine therapies in patients with breast cancer. Accumulating clinical data suggest adverse outcome for patients harboring tumors expressing these mutations. We aimed to elucidate mechanisms conferring this aggressive phenotype. Experimental Design: Cells constitutively expressing physiologic levels of ER-harboring activating LBD mutations were generated and characterized for viability, invasiveness, and tumor formation in vivo. Gene expression profile was studied using microarray and RNAseq technologies. Metabolic properties of the cells were assessed using global metabolite screen and direct measurement of metabolic activity. Results: Cells expressing mutated ER showed increased proliferation, migration, and in vivo tumorigenicity compared with cells expressing the wild-type ER (WT-ER), even in the presence of estrogen. Expression of the mutated ER was associated with upregulation of genes involved in invasion and metastases, as well as elevation of genes associated with tumor cell metabolism. Indeed, a metabolic examination revealed four distinct metabolic profiles: WT-ER–expressing cells either untreated or estrogen treated and mutated ER–expressing cells either untreated or estrogen treated. Pathway analyses indicated elevated tricarboxylic acid cycle activity of 537S-ER–expressing cells. Thus, while WT-ER cells were mostly glucose-dependent, 537S-ER were not addicted to glucose and were able to utilize glutamine as an alternative carbon source. Conclusions: Taken together, these data indicate estrogen-independent rewiring of breast cancer cell metabolism by LBD-activating mutations. These unique metabolic activities may serve as a potential vulnerability and aid in the development of novel treatment strategies to overcome endocrine resistance.