REGULATION OF GLUCOCORTICOID RECEPTOR BY PHOSPHORYLATION

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The overall objectives of this proposal are to characterize the molecular mechanisms by which phosphorylation regulates glucocorticoid receptor (GR) activity. GR is a hormone-dependent transcription factor expressed in virtually all tissues, yet it displays a remarkable capacity to regulate genes in a cell type- specific manner. Although glucocorticoids act as the primary signal in activating GR's transcriptional regulatory functions, GR-mediated transcriptional activity is also regulated by phosphorylation. The amino terminus of GR contains a transcriptional activation domain that is phosphorylated at four major sites in cultured mammalian cells. Several kinases have been identified that phosphorylate GR in vitro at the identified sites. Of these, the cyclin-dependent kinases (Cdks) phosphorylate serine 232 (S232) and serine (S224), while c-Jun N- terminal kinase (JNK) phosphorylates serine 246 (S246) and glycogen synthase kinase-3 (GSK3) phosphorylates threonine 171 (T171). Phosphorylation of these sites is important for GR function: serine to alanine mutations of S224 and S232 decrease GR transcriptional activation, whereas alanine mutations of T171 and S246 increase GR transcriptional activation. Thus, GR- mediated transcriptional activity is regulated both positively and negatively by phosphorylation. We propose that phosphorylation by multiple protein kinases enables GR to respond to diverse extracellular signals. This ability to integrate multiple signals in the form of phosphorylation permits a flexibility in GR action that, in conjunction with the steroid ligands, may be crucial in coordinating the cell type specific actions of GR. We further hypothesize that phosphorylation regulates GR's interaction with proteins involved in transcriptional regulation. This hypothesis will be addressed by expressing activators or inhibitors of Cdk, JNK and GSK3 in cultured mammalian cells in transient transfection assays designed to monitor GR-dependent transcriptional regulation. In addition, we will identify and characterize proteins that interact with the GR N-terminal transcriptional activation domain in a phosphorylation-dependent manner using a protein interaction screen in yeast. Gaining a mechanistic understanding of the communication between multiple signaling pathways, as realized through GR and its regulatory kinases, is fundamental to understanding the mechanism of GR-regulated gene expression and may reveal likely points of intervention to be exploited in the development of new therapies for glucocorticoid-resistant malignancies, such as breast cancer and acute lymphoblastic leukemia.