Serine / threonine protein phosphatase 5 (PP5) participates in the regulation of glucocorticoid receptor nucleocytoplasmic shuttling

To further characterize the mechanism by which PP5 affects GR-induced gene expression, we employed immunofluorescence microscopy to track the movement of a GR-green fluorescent fusion protein (GR-GFP) that retained hormone binding, nuclear translocation activity and specific DNA binding activity, but is incapable of transactivation. In the absence of glucocorticoids, GR-GFP localized mainly in the cytoplasm. Treatment with dexamethasone results in the efficient translocation of GR-GFPs into the nucleus. The nuclear accumulation of GR-GFP, without the addition of glucocorticoids, was also observed when the expression of PP5 was suppressed by treatment with ISIS 15534. In contrast, ISIS 15534 treatment had no apparent effect on calcium induced nuclear translocation of NFAT-GFP.These studies suggest that PP5 participates in the regulation of glucocorticoid receptor nucleocytoplasmic shuttling, and that the GR-induced transcriptional activity observed when the expression of PP5 is suppressed by treatment with ISIS 15534 results from the nuclear accumulation of GR in a form that is capable of binding DNA yet still requires agonist to elicit maximal transcriptional activation.Glucocorticoids influence a wide spectrum of cellular functions through their action on soluble intracellular receptors. In most cells, unliganded glucocorticoid receptors (GR) reside predominately in the cytoplasm, where they exist as a heteromeric complex comprised minimally of GR, 90-kDa and 70-kDa heat shock proteins (hsp90 and hsp70). Other proteins (i.e. p60/Hop, p23, hsp40, FKBP52, and FKBP51) have been implicated in the assembly/stabilization of the GR-hsp90-hsp70-complex in a form that has high affinity for agonist [for review, see Ref. 1, 2, 3]. Upon agonist binding, the complex undergoes a transformation, and the ligand bound GR translocates into to the nucleus in a manner that is determined by a nuclear localization sequence (NLS) contained in the receptor [4]. There the GR acts as a liga