Evaluation of Post-Transcriptional Regulation of Glucocorticoid Receptor in Multiple Myeloma and the Role of MicroRNA

Multiple myeloma (MM) is a malignancy of plasma cells and despite recent advances in therapy remains incurable. Glucocorticoids (GCs) have been a mainstay of treatment for many years due to the ability of these drugs to induce apoptosis. Unfortunately, the efficacy of GCs is limited by the development of resistance that occurs in all patients. The mechanism of resistance is not completely understood, however it is correlated with decreased expression of the glucocorticoid receptor (GR). GR expression levels in MM patients are positively correlated with survival. The MM.1 cell line model is widely used in MM research and is ideal for studying GC actions. Within this model system, MM.1S cells are sensitive to GCs and express GR, while MM.1Re and MM.1RL are resistant to GCs and express low or undetectable levels of GR protein, respectively. Previous studies established that differences in GR expression in these cell lines of shared genetic origin are due to post-transcriptional regulation. In this thesis several of these mechanisms were investigated including alternative splicing, proteasome degradation, alternative translation, and regulation by microRNAs (miRNAs). Alternative splicing proteasome degradation and alternative translation were excluded as principal regulators of GR in myeloma. MiRNA were selected as a likely candidate for GR regulation based on decreased luciferase activity in MM.1R cells compared to MM.1S in the presence of a luciferase reporter carrying the 3'UTR of GR. Through a comprehensive screening and validation process, one miRNA in particular, miR-130b, was identified to repress GR expression. Furthermore, overexpression of this miRNA desensitized MM.1S cells to GC induced apoptosis. Expression of miR-130b has the potential to serve as a biomarker for patients who could become refractory to GCs. Furthermore, these findings represent the first documentation of miRNA regulation of GR in MM. As more is learned about additional miRNA regulation of this pathway, there is increased potential for clinical intervention to restore GC sensitivity in MM patients.