Differential distribution of glucocorticoid and estrogen receptor isoforms: localization of GRbeta and ERalpha in nucleoli and GRalpha and ERbeta in the mitochondria of human osteosarcoma SaOS-2 and hepatocarcinoma HepG2 cell lines

The localization of glucocorticoid and estrogen receptors alpha (GRalpha, ERalpha) and beta (GRbeta, ERbeta) in osteosarcoma SaOS-2 and hepatocarcinoma HepG2 cells was studied by immunofluorescence labelling and confocal laser scanning microscopy, as well as by subcellular fractionation and immunoblotting of the proteins of the fractions with respective antibodies. In HepG2 and SaOS-2 cells GRbeta and ERalpha were localized mainly in the nucleus, particularly concentrated in nuclear structures, which on the basis of their staining with antibody against C23-nucleolin, were characterized as nucleoli. A faint, diffuse GRbeta and ERalpha staining was also observed in the cytoplasm. GRalpha and ERbeta were specifically enriched at the site of cell mitochondria, which were visualized by labelling with the vital dye CMX. Immunoblotting experiments corroborated the immunofluorescence labelling distribution of glucocorticoid and estrogen receptor isoforms in the cell lines studied. These findings support the concept of a direct action of steroid/thyroid hormones on mitochondrial functions by way of their cognate receptors and also suggest a direct involvement of GRbeta and ERalpha in nucleolar-related processes in HepG2 and SaOS-2 cells.