The role of the newly discovered steroid receptor RNA activator protein (SRAP) in the estrogen signaling pathway and its implication in breast cancer
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In 1999, the discovery of the Steroid Receptor RNA Activator (SRA) was unprecedented in the field of steroid receptor co-regulator research. It was the first time that an RNA molecule was demonstrated to function similarly to its protein counterpart and modulate the activity of steroid receptors. This peculiar steroid receptor co-activator thus attracted the attention of numerous research groups. Over the years, studies were reported deciphering SRA mechanisms of action, its role in co-regulating nuclear receptors and its possible implication in human diseases. While SRA was originally thought to exist solely as a non-coding RNA, our laboratory has identified longer SRA RNA isoforms with the theoretical capacity to encode for a protein. This discovery impelled us to investigate the existence of a Steroid Receptor RNA Activator Protein or SRAP. In this thesis, we first demonstrated the existence and function of endogenous evolutionary conserved SRA proteins. Based on these results we further explored SRAP expression in breast tumors. Interestingly, Western blot analysis of a small cohort of estrogen positive breast tumors suggested that SRAP expression correlates with a better overall survival in patients treated with tamoxifen. This observation prompted us to explore the biological role of SRAP. We found that MCF-7 cells stably expressing coding SRA isoforms had lower ligand dependent estrogen receptor alpha transcriptional activity. In order to dissect the function of the protein independently of its RNA counterpart, we separated the functions of the protein by introducing extensive silent mutations into the RNA sequence. Using this model, we established that SRAP, independent of its RNA counterpart, enhances estrogen receptor alpha activity in a ligand and response-element dependent manner. Furthermore, we showed for the first time that SRAP physically interacts with multiple transcription factors and is recruited to specific promoter regions. Moreover, by artificially recruiting SRAP to the promoter of a luciferase reporter gene under the control of the strong transcriptional activator VP16, we observed a decrease in transcription. These latter results suggest that SRA could function as a repressor through direct association with promoters. Overall, we believe that SRA is a very peculiar example of a bi-faceted system consisting of a functional RNA and its corresponding protein. Altogether our data suggest that SRAP, similarly to its RNA counterpart, is involved in many critical pathways that directly participate in gene expression regulation.