To reduce the side effects of corticosteroid treatment, the administered dose of glucocorticoids (GCs) should be kept to a minimum while preserving therapeutically needed intracellular levels. Currently available assays to determine individual sensitivity to GCs are either imprecise or based on inhibition by GCs of lymphocyte proliferation following stimulation with phytohemagglutinin or other mitogens, which may influence the GC signal transduction pathway. Using the human lymphoblastoid cell line IM-9 as a model system, we studied whether the GC-induced increase of the mRNA encoding the 51-kDa FK506-binding protein (FKBP51) could be used for the development of a novel assay, ultimately to be used in native human peripheral blood mononuclear cells (PBMCs). GC addition to IM-9 cells resulted in a dose-dependent increase of FKBP51 mRNA levels within 2 h, followed by a further increase until 24 h. Northern blot analysis and real-time PCR yielded similar results. Coincubation of GCs with the GC receptor antagonist ORG 34116 or the protein synthesis inhibitor cycloheximide suggested a direct, GC receptor-mediated up-regulation of FKBP51 gene transcription. Expected differences in potency among different GCs could be readily demonstrated in this system. Extending our observations in IM-9 lymphoblasts to normal PBMCs, we found a dose-dependent increase of FKBP51 mRNA on ex vivo incubation of native human PBMCs with GCs, with a sensitivity of about 10(-9) M for dexamethasone. Moreover, dexamethasone ingestion increased FKBP51 mRNA in PBMCs in vivo, extending the use of this assay to the measurement of GC bioavailability. Finally, using this method we were able to demonstrate partial GC-insensitivity in a 6-month-old infant suffering from congenital adrenal hyperplasia caused by 21-hydroxylase deficiency. We conclude that the induction of FKBP51 mRNA by GCs may be a suitable marker to assess individual GC sensitivity, the in vitro measurement of GC potency, and the in vivo determination of GC bioavailability.