Intranuclear dynamics of corticosteroid receptors and effects of proteasomal activity in cultured hippocampal neural cells.
Adrenal corticosteroids readily enter the brain and exert markedly diverse effects, such as stress responses in the target neural cells. These effects are regulated by two receptor systems via the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), which are both ligand-dependent transcription factors. Several steroid hormone receptors including GR, estrogen receptor, and androgen receptor, have been shown to move rapidly in the nucleus even after ligand treatment, supposedly corresponding to transcriptional "fine-tuning". We applied fluorescence recovery after photobleaching (FRAP) to assess the mobility of green fluorescent protein (GFP)-tagged GR and -MR in the nucleus of transiently transfected cultured hippocampal neurons. FRAP results showed high mobility of GR and MR in the nucleus. Half-recovery time of GR was longer than that of MR in the presence of 10(-6)M corticosterone (CORT), but shorter in the presence of 10(-9)M CORT. Proteasome inhibition reduced the subnuclear mobility of GR and MR, and increased the transcriptional activity at both concentrations of CORT. We also investigated the differential effects of CORT concentration and proteasome inhibition on the nuclear retention level of these receptors. Our findings may provide intriguing new insights into the dynamics of corticosteroid receptors in neural cells and the molecular basis of stress regulation by these receptors in the hippocampus.