Wanted: A New Model for Glucocorticoid Receptor Transactivation and Transrepression

Glucocorticoid receptor (GR) is a transcription factor that is almost ubiquitous in vertebrate cells. As one of the cellular receptors for steroid hormones of the glucocorticoid (GC) family, which are well known for their anti-inflammatory properties in clinical applications, the regulation of GR activity has been the focus of intense study. Upon binding GC, GR translocates from the cytoplasm to the nucleus, where it may either bind to and activate transcription from gene promoters (transactivation) or interact with other transcription factors to alter their function (transrepression). These two modes have very different effects, and GR transactivation is linked to many of the negative side effects of GCs that are used therapeutically. Several groups are therefore searching for GR ligands that can separate GR’s transactivation and transrepression activities. However, the search is hampered because the mechanisms of GR transactivation and transrepression are poorly understood. In this month’s PLOS Biology, Diego M. Presman, Adali Pecci, Gordon L. Hager, and colleagues report on their efforts to clarify GR’s different modes of activity. GR can form a dimer with itself, and one popular model holds that the GR dimer transactivates genes, whereas monomeric GR participates in transrepression. This suggestion was based on earlier work with a mutant version of GR called GRdim, which was reported to participate in transrepression, but not transactivation, and was thought to be deficient in dimerization. However, newer research has challenged these findings, so Presman et al. sought to determine whether GR dimerization is required for transactivation. Presman et al. used a microscopy technique called the Number and Brightness (N&B) assay, which can detect whether a fluorescently tagged protein has dimerized by measuring pixel-intensity fluctuations in captured images of cultured cells (Figure 1). As expected, in untreated mouse or hamster cells expressing Green Fluorescent Proteintagged GR, GR was mostly found in the cytoplasm. Further, treatment with a GR ligand caused GR to translocate to the nucleus, where N&B detected its dimerization. However, N&B also detected dimerization of GRdim following ligand exposure, indicating that this mutant can dimerize after all. Besides its purported inability to dimerize, GRdim was thought to be incapable of transactivation because it could not bind to DNA. Presman et al. tested this in vivo using a cell line that contains an artificial stretch of DNA with a tandem array of GR-binding sites and found that, like wildtype GR, GRdim could bind to this array. In addition, when the authors examined