Purpose: We hypothesized that estrogen receptor-a (ER-a) status in endometrial carcinomas, associated with poor prognosis, is reflected in transcriptional signatures suggesting targets for new therapy.Experimental Design: Endometrial carcinoma samples in a primary investigation cohort (n ¼ 76) and three independent validation cohorts (n ¼ 155/286/111) were analyzed through integrated molecular profiling. Biomarkers were assessed by immunohistochemistry (IHC), DNA oligonucleotide microarray, quantitative PCR (qPCR), single-nucleotide polymorphism (SNP) array, and Sanger sequencing in the cohorts, annotated for comprehensive histopathologic and clinical data, including follow-up.Results: ER-a immunohistochemical staining was strongly associated with mRNA expression of the receptor gene (ESR1) and patient survival (both P < 0.001). ER-a negativity associated with activation of genes involved in Wnt-, Sonic Hedgehog-, and TGF-b signaling in the investigation cohort, indicating epithelial-mesenchymal transition (EMT). The association between low ER-a and EMT was validated in three independent datasets. Furthermore, phosphoinositide 3-kinase (PI3K) and mTOR inhibitors were among the top-ranked drug signatures negatively correlated with the ER-a-negative tumors. Low ER-a was significantly associated with PIK3CA amplifications but not mutations. Also, low ER-a was correlated to high expression of Stathmin, a marker associated with PTEN loss, and a high PI3K activation signature.Conclusion: Lack of ER-a in endometrial cancer is associated with EMT and reduced survival. We present a rationale for investigating ER-a's potential to predict response to PI3K/mTOR inhibitors in clinical trials and also suggest EMT inhibitors to ER-a-negative endometrial carcinomas.