Protein 14-3-3 sigma interacts with and favors cytoplasmic subcellular localization of the glucocorticoid receptor, acting as a negative regulator of the glucocorticoid signaling pathway

The glucocorticoid receptor (GR) alpha interacts with the highly conserved 14-3-3 family proteins. The latter bind phosphorylated serine/threonine residues of "partner" molecules and influence many signal transduction events by altering their subcellular localization and/or protecting them from proteolysis. To examine the physiologic role of 14-3-3 on the glucocorticoid-signaling pathway, we studied the nucleocytoplasmic shuttling and transactivation properties of GRalpha in a cell line replete with or devoid of 14-3-3sigma. We found that endogenous 14-3-3sigma helped localize green fluorescent protein-fused GRalpha in the cytoplasm in the absence of ligand and potentiated its nuclear export after ligand withdrawal. 14-3-3sigma also suppressed the transcriptional activity of GRalpha on a glucocorticoid- responsive promoter. Disruption of the classic nuclear export signal of 14-3-3sigma inactivated its ability to influence the nucleocytoplasmic trafficking and transactivation activity of GRalpha, whereas introduction of a mutation inactivating the binding activity of 14-3-3sigma to some of its partner proteins did not. 14-3-3sigma bound the ligand-binding domain of GRalpha through its COOH-terminal portion, in a partially ligand- dependent fashion, while it did not interact with "ligand- binding domain" of GRbeta at all. These results suggest that 14-3-3sigma functions as a negative regulator in the glucocorticoid signaling pathway, possibly by shifting the subcellular localization/circulation of this receptor toward the cytoplasm through its nuclear export signal. Since 14-3-3 proteins play significant roles in numerous cellular activities, such as cell cycle progression, growth, differentiation, and apoptosis, these actions might indirectly influence the transcriptional activity of GRalpha. Conversely, through its 14-3-3 protein interactions, GRalpha may influence these processes.

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