Regulation of glucocorticoid receptor nuclear localization in prostate cancer cells.

Glucocorticoid receptor (GR) plays important roles in many diseases including prostate cancer. Intracellular shuttling of GR is thought to be an important mechanism regulating its localization to the nucleus required for transactivation of GR target genes. Here, using fluorescent microscopy coupled with pulse-chase and nucleocytoplasmic fractionation coupled with western blot, we provided evidence that GR can be imported and then degraded in the nucleus in the absence of ligand. We also showed that nuclear GR was stabilized by glucocorticoid hormone and that hormone withdrawal caused nuclear GR degradation, but not export. Further analysis showed that GR ubiquitination occurred predominantly in the nucleus compared with cytoplasm and was suppressed by glucocorticoids. Using small interfering RNA knockdown, we showed that loss of E3 ligase CHIP significantly inhibited GR ubiquitination and degradation in the nucleus, while enhancing the expression of GR target gene SGK1. These findings support an updated model that GR nucleocytoplasmic trafficking is a 1-way trip, involving nuclear import but not export. Future studies should focus on defining the mechanisms regulating GR ubiquitination and degradation in the nucleus, which may lead to novel approaches to modulate GR function for disease treatment. SIGNIFICANCE STATEMENT: This study suggests that glucocorticoid receptor (GR) nucleocytoplasmic trafficking is a 1-way trip, involving nuclear import but not export. This will guide future studies on defining the mechanisms regulating GR nuclear localization, which may lead to novel approaches to modulate GR function for disease treatment.