RNA-Binding Protein Musashi-2 Inhibits Aldosterone Production

The adrenal cortex is the site of steroid hormone synthesis. These hormones control important physiological processes like metabolism, blood pressure and volume, and sexual characteristic development. While the signaling pathways, transcription factors, and steroidogenic enzymes are well-characterized, surprisingly little is known about the contribution RNA-binding proteins (RBPs). RBPs exert post-transcriptional control by interacting with specific elements within target mRNAs. Here we focus on the RBP, Mushashi-2 (MSI2), which binds to UAG sequences in the 3’UTR of its target transcripts. MSI2 is required for development of steroidogenic tissues which is consistent with its higher mRNA levels in human ovaries and testis. MSI2 also exhibits high expression levels in human adrenal tissue and the immortalized human adrenocortical cell line (H295R). Based on the compelling MSI2 expression pattern, we set out to determine the role of MSI2 on aldosterone production. Depletion of MSI2 using siRNA led to significantly lower aldosterone levels in H295R cells stimulated with AngII. We also employed an orthogonal loss-of-function approach by co-treating cells with AngII and increasing concentrations of Ro-08-2750 (Ro), a direct and selective inhibitor of MSI2-RNA interactions. Ro inhibited aldosterone production in a dose-dependent manner at 1 µM with almost complete inhibition at 5 µM. The molecular mechanism by which MSI2 regulates target RNA translation and/or decay is unknown. Moreover, whether MSI2 acts as a repressor or activator appears to be context dependent. Our goal is to determine the precise molecular mechanism by which MSI2 promotes aldosterone production. Specifically, we will identify MSI2 targets, temporally resolved consequences of MSI2 inhibition, and protein interaction partners. This work will impact our understanding of fundamental principles of RBP-mediated regulation, as well as novel regulatory mechanisms underlying human steroid hormone synthesis. Indeed, Ro (or further optimized compounds) may represent new therapeutic avenues for adrenal disease.