Ursodeoxycholic acid induced activation of the glucocorticoid receptor in primary rat hepatocytes

Ursodeoxycholic acid (UDCA), a hydrophilic bile acid, improves biochemical, immunopathological and histological parameters in chronic cholestatic liver diseases. The immunomodulatory properties of UDCA show interesting similarities with the effects of glucocorticoids. We investigated the activation of the glucocorticoid receptor by UDCA and the glucocorticoid receptor dependent gene expression in primary rat hepatocytes as well as binding of radiolabelled UDCA to the glucocorticoid receptor ligand binding site expressed in a glucocorticoid receptor fusion protein.

Primary rat hepatocytes in culture were co-transfected with a luciferase reporter gene construct (GRE-luc) containing a glucocorticoid receptor responsive element (GRE) and a glucocorticoid receptor expression vector (6RGR) followed by stimulation with dexamethasone or UDCA. Luciferase activity was determined and specific binding of glucocorticoid receptor to the GRE was confirmed by an electrophoretic mobility shift assay (EMSA). The glucocorticoid receptor binding site was expressed in a GR-myc fusion protein and binding of radiolabelled UDCA to the fusion protein was determined.

Incubation of co-transfected hepatocytes with 0.1-1.000 microM dexamethasone or 0.1-1.000 microM UDCA led to an 11.9- to 20.85-fold (dexamethasone) and 2.6- to 4.3-fold (UDC) increase of luciferase activity. Mobility shift assays using nuclear extracts from transfected and stimulated hepatocytes also showed a dose dependent increase of DNA binding after stimulation with UDCA. However, incubation of the GR-myc fusion protein with radiolabelled UDCA yielded no specific binding of UDCA to the glucocorticoid receptor binding site, whereas dexamethasone showed specific binding of the fusion protein.

UDCA activates the intracellular glucocorticoid receptor in a dose-dependent manner. Direct binding of the glucocorticoid receptor by radiolabelled UDCA at the glucocorticoid receptor binding site could be excluded as the mechanism of activation. The mechanisms involved in UDCA-mediated glucocorticoid receptor activation and possible targeted glucocorticoid receptor activation due to partial UDCA tissue specificity warrant further elucidation.