| Summary: | Glucocorticoid receptor (GR) is a ligand-dependent transcription factor, involved in the regulation of hundreds of genes. In the absence of any ligand, GR resides in the cytoplasm where it is sequestered in a multimeric chaperone complex consisting of hsp90, hsp70, p23, Hop, FKBP51, FKBP52, etc. As part of this multiprotein complex, GR undergoes conformational changes that allow glucocorticoid hormone binding. Upon ligand binding, GR dissociates from chaperon complex and translocates into the nucleus, where it interacts with specific DNA sequences (GREs) in the regulatory regions of target genes and modulates their expression. Then unliganded GR is exported to the cytoplasm, completing the nuclear-cytoplasmic cycle. Recent evidence suggests that, in addition to this cycle, chromatin and chaperone GR cycles exist within the nuclei. The chromatin cycle implies repeated interactions of ligand-bound GR with GREs in the chromatin context lasting for few seconds. The chaperone cycle starts after dissociation of the hormone–receptor complex, when GR binds to the nuclear chaperone machinery. As a result, its hormone-binding affinity is regained. Upon hormone binding, GR releases from chaperon complex and binds to GREs again. It is assumed that the chaperone cycle is mainly responsible for prolonged GR retention in nuclei (half-life within 8–12 h upon steroid withdrawal). In this review, we summarize and critically analyze the published data on chromatin and intranuclear chaperone GR cycles.
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