Regulation of glucocorticoid receptor by the tyrosine kinase inhibitor herbimycin A in the cytosolic fraction of primary cultured rat hepatocytes.

The participation of tyrosine kinase in the regulation of the glucocorticoid receptor (GR) was studied in primary cultured rat hepatocytes using the tyrosine kinase inhibitor herbimycin A. Herbimycin A decreased the number of high-affinity binding sites of glucocorticoids in the cytosolic fraction and increased the equilibrium dissociation constant (Kd). Western blot analysis revealed that it also decreased the amount of GR protein. On the other hand, cycloheximide did not affect the GR protein level. Although herbimycin A slightly increased the amount of GR protein in the nuclear fraction, the increase was much lower than that of its decrease in the cytosolic fraction. Therefore, the decrease of GR protein in the cytosolic fraction was not caused by the inhibition of GR protein synthesis nor the translocation of GR from cytosol to nuclei. As herbimycin A also suppressed the dexamethasone (Dex)-dependent induction of tyrosine aminotransferase (TAT) activity, the decrease of GR protein was followed by the suppression of the GR-mediated biological response. These findings indicate that tyrosine kinase is necessary for the maintenance of the level of GR protein and its affinity of binding sites in the cytosolic fraction. Our results also suggested that the increase of GR protein stability is the most probable explanation for the maintenance of its level.