Glucocorticoid receptor isoforms in human hepatocarcinoma HepG2 and SaOS-2 osteosarcoma cells: Presence of glucocorticoid receptor alpha in mitochondria and of glucocorticoid receptor beta in nucleoli

In the context of a possible direct action of glucocorticosteroids on mitochondrial transcription and/or apoptosis by way of cognate mitochondrial receptors, the possible localization of glucocorticoid receptors α and β (GRα and GRβ) in mitochondria was explored in human hepatocarcinoma HepG2 and osteosarcoma SaOS-2 cells, in which glucocorticoids exert an anabolic and apoptotic effect, respectively. In both cell types, GRα was detected in mitochondria, in nuclei and in cytosol by immunofluorescence labeling and confocal scanning microscopy, by immunogold electron microscopy and by Western blotting. GRβ was shown to be almost exclusively restricted to the nucleus of the two cell types, being particularly concentrated in nucleoli, pointing to a solely nuclear role of this receptor isoform and to a possible function in nucleoli related processes. Computer analysis identified a putative internal mitochondrial targeting sequence within the glucocorticoid receptor. The demonstration of mitochondrially localized GRα in HepG2 and SaOS-2 cells corroborates previous findings in other cell types and further supports a direct role of this receptor in mitochondrial functions.