Endocrine Abstracts

The actions of glucocorticoids (Gc) are mediated by the glucocorticoid receptor (GR). Several isoforms of the GR exist. Whilst GR alpha is the most abundant isoform, constitutive alternative splicing at the exon 3/exon 4 boundary results in an additional arginine within the DNA binding domain to produce the gamma isoform. Although GR gamma comprises 4–8% of total GR transcripts and is highly conserved through mammalian evolution, its biological function remains unknown.

Analysis of GR localisation in unstimulated HeLa and U20S cells using fluorophore-tagged constructs demonstrates a markedly different baseline distribution of the two isoforms, where GR gamma has almost exclusively cytoplasmic distribution, and GR alpha displays a more heterogeneous distribution. Live cell imaging, and high throughput analysis of fixed cells show that the kinetics of trafficking following ligand treatment is also very different, since GR gamma translocates to the nucleus at a significantly slower rate than GR alpha.

Analysis of membrane localised GR reveals that whilst GR alpha rapidly translocates into the nucleus following addition of Gc, GR gamma remains at the cell surface for several minutes before nuclear translocation. GR gamma also mediates enhanced Gc-induced activation of JNK, compared to GR alpha, which together with prolonged cytoplasmic residence may suggest a distinct role for GR gamma.

GR gamma differentially transactivates a panel of reporter genes in a reporter and cell-specific manner and consistently fails to transrepress. Preliminary microarray data further supports distinct regulatory roles for GR alpha and gamma on target genes since GR alpha gene regulatory effects are almost exclusively ligand-dependent and the effects GR gamma appear largely ligand-independent.

GR gamma therefore likely subserves a distinct spectrum of biological effects in the nucleus and cytoplasm of Gc target cells. This has implications for Gc action in health and disease.