Defective glucocorticoid receptor nuclear translocation and altered histone acetylation patterns in glucocorticoid-resistant patients

Most chronic inflammatory diseases are well controlled by glucocorticoids. However, a minority of patients fails to respond adequately to this treatment. We wished to determine whether glucocorticoid insensitivity in a group of steroid-resistant (SR) and steroid-dependent (SD) asthmatic subjects resulted from an inability of the glucocorticoid receptor (GR) to translocate into the nucleus. Glucocorticoid receptor nuclear translocation was determined in PBMCs by immunocytochemistry and GR function measured by suppression of TNF-α–induced GM-CSF release and effects of dexamethasone on histone acetylation. Glucocorticoid repression of TNF-α–induced GM-CSF release was reduced in PBMCs from SD and SR patients. This inhibition correlated with a failure of GR to translocate into the nucleus and induce histone acetylation in response to dexamethasone. In addition, dexamethasone failed to inhibit TNF-α–induced histone acetyltransferase activity, which predominantly targeted histone residues lysine (K)8 and K12. However, in a subset of patients, even high levels of GR nuclear translocation failed to produce histone acetylation in response to dexamethasone. Histone H4 K5 acetylation, a marker of dexamethasone transactivation, was specifically reduced in this group. However, cells from this subset of steroid-insensitive subjects were still capable of inhibiting TNF-α–induced histone acetylation. We have identified a novel mechanism of glucocorticoid insensitivity in a group of SR and SD subjects. These data suggest that most patients respond to glucocorticoids according to the degree of GR nuclear translocation occurring, but some subjects with steroid resistance have a reduced response because of a failure of steroids to transactivate, rather than transrepress.