THE UNLIGANDED GLUCOCORTICOID RECEPTOR IS LOCALIZED IN THE NUCLEUS, NOT IN THE CYTOPLASM

The glucocorticoid receptor (GR) is often described to be localized in the cytoplasm in the absence of hormone and to translocate to the nucleus upon binding of the hormone. This apparently different behavior of the GR compared to that of other members of the steroid receptor superfamily is unexpected because similarities in the molecular structures of steroid hormone receptors would predict similarities in their working mechanisms. The absence of the unliganded GR from the nuclear compartment may be due to an artefactual redistribution, occurring during the immunocytochemical procedure. We systematically studied the effects of various fixation and permeabilization procedures on the distribution of the GR in hepatoma cells that were incubated in steroid-free or steroid-containing medium. Immunofluorescent labeling of the GR in formaldehyde-fixed and detergent-permeabilized cells resulted in the almost complete absence of immunoreactivity of cells when the receptor was in its unliganded form, whereas the liganded receptor was detected mainly in the nucleus. On the other hand, labeling of cryosectioned glutaraldehyde/formaldehyde-fixed cells demonstrated that receptor antigenicity is present in the nucleus in the absence as well as the presence of steroid. We conclude that the results obtained with the cryosectioning procedure reflect the receptor distribution in the living cell, since glutaraldehyde fixation of the cells prevented the washout of loosely bound receptor. The predominantly nuclear location of the unliganded GR in hepatoma cells and the absence of changes in distribution after steroid stimulation imply that the nuclear translocation model is not true for the GR.