Endocrine Abstracts

Glucocorticoids (Gcs) act via the intracellular glucocorticoid receptor (GR) to regulate cellular homeostasis. GR is a ligand-activated transcription factor that mediates effects within the nucleus or cytoplasm to regulate genomic and non-genomic events. Localisation of the GR to a specific cellular compartment is therefore an important determinant of the cellular response to Gcs.

Live cell imaging of GR trafficking using fluorophore-tagged GR shows dramatic GR export from the nucleus during mitosis, even in the presence of the synthetic Gc dexamethasone (dex). High throughput analysis of fixed cells demonstrates this phenomenon is robust and specific to mitosis.

Immunoblotting shows a shift in the kinetics of dex-mediated activation of MAPK in mitotic cells, which correlates with a larger proportion of cytoplasmic GR available to mediate signalling. Reporter studies demonstrate selective impairment of transactivation but not transrepression in mitosis, suggesting specific regulation of GR function by mitosis.

Treatment with the nuclear export inhibitor LMB only partially blocked mitotic export of GR. Inhibitors to both JNK and p38 also had a similar effect on GR trafficking. However, incubation with LMB, JNK or p38 inhibitors did not restore dex-induced GR transactivation in mitotic cells.

An increase in ligand-independent ser211 phosphorylation of GR was observed in mitosis. Since there was also a significant increase in ligand-independent GR transactivation this might suggest a link between GR phosphorylation, subcellular GR trafficking and transactivation in mitosis.

Cell cycle phase therefore regulates ligand-independent GR localisation and selectively impacts GR activity both on the rapid signalling events in the cytoplasm and also the longer acting gene regulatory events within the nucleus. We therefore correlate the sensitivity of cells to Gcs with their mitotic index which in turn may have implications for Gc action in rapidly dividing cells.