The dynamics of intranuclear movement and chromatin remodeling by the glucocorticoid receptor

Acetylation-mediated epigenetic regulation of glucocorticoid receptor activity: circadian rhythm-associated alterations of glucocorticoid actions in target tissues

Glucocorticoids influence organ functions through the glucocorticoid receptor, a protein acetylated and deacetylated by several histone acetyltransferases and deacetylases. We reported that the circadian rhythm-related transcription factor "Clock", a key component of the biological CLOCK with inherent histone acetyltransferase activity, acetylates glucocorticoid receptor lysines within its hinge region--a "lysine cluster" containing a KXKK motif--and represses its transcriptional activity. This Clock-induced repression of the glucocorticoid receptor activity is inversely phased to the diurnally circulating glucocorticoids and may act as a local counter regulatory mechanism to the actions of these hormones. Importantly, uncoupling of the central CLOCK-regulated hypothalamic-pituitary-adrenal axis and peripheral CLOCK-mediated alterations of glucocorticoid action, such as chronic stress and frequent trans-time zone travel or night-shift work, may cause functional hypercortisolism and contribute to various pathologies. Thus, acetylation-mediated epigenetic regulation of the glucocorticoid receptor may be essential for the maintenance of proper time-integrated glucocorticoid action, significantly influencing human well-being and longevity.

Dynamic access of the glucocorticoid receptor to response elements in chromatin

Transcriptional activation as a rate-limiting step of gene expression is often triggered by an environmental stimulus that is transmitted through a signaling cascade to specific transcription factors. Transcription factors must then find appropriate target genes in the context of chromatin. Subsequent modulation of local chromatin domains is now recognized as a major mechanism of gene regulation. The interactions of transcription factors with chromatin structures have recently been observed to be highly dynamic, with residence times measured in seconds. Thus, the concept of static, multi-protein complexes forming at regulatory elements in the genome has been replaced by a new paradigm that envisages rapid and continuous exchange events with the template. These highly dynamic interactions are a property of both DNA-protein and protein-protein interactions and are inherent to every stage of the transcriptional response. In this review we discuss the dynamics of a nuclear receptor, and its transcriptional response in the chromatin context.

Chinese hamster ORC subunits dynamically associate with chromatin throughout the cell-cycle

In yeast, the Origin Recognition Complex (ORC) is bound to replication origins throughout the cell-cycle, but in animal cells, there are conflicting data as to whether and when ORC is removed from chromatin. We find ORC1, 2 and ORC4 to be metabolically stable proteins that co-fractionate with chromatin throughout the cell-cycle in Chinese hamster fibroblasts. Since cellular extraction methods cannot directly examine the chromatin binding properties of proteins in vivo, we examined ORC:chromatin interactions in living cells. Fluorescence loss in photobleaching (FLIP) studies revealed ORC1 and ORC4 to be highly dynamic proteins during the cell-cycle with exchange kinetics similar to other regulatory chromatin proteins. In vivo interaction with chromatin was not significantly altered throughout the cell-cycle, including S-phase. These data support a model in which ORC subunits dynamically interact with chromatin throughout the cell-cycle.