One Hormone for Two Receptors: Exploring Glucocorticoid Actions Mediated by the Glucocorticoid and Mineralocorticoid Receptors

Glucocorticoids are indispensable for mediating the response to stress, energy demands, development, and limiting inflammation. Once in the cell, these hormones exert their actions by activating nuclear receptors, transcription factors that regulate gene expression. The glucocorticoid receptor (GR) is the transcription factor that predominantly mediates both physiological and pharmacological glucocorticoid effects. Yet glucocorticoids can also bind and activate the mineralocorticoid receptor (MR), a transcription factor known to bind aldosterone thus maintaining whole-body fluid homeostasis. Phylogenetically, GR and MR are closely related and share a remarkable structural similarity. Indeed, the DNA-binding domain of MR is 96% identical to that of GR; thus MR is recruited to many of the same DNA response elements that bind GR. Moreover, GR has a low affinity for glucocorticoids but is expressed in nearly every cell, whereas MR shows a higher affinity for glucocorticoids although knowledge of MR’s expression levels is somewhat limited. These characteristics suggest that, while GR and MR can compensate for each other’s actions in many tissues, there are specific glucocorticoid and mineralocorticoid-mediated responses indicating GR-MR functional diversity. To investigate the similarities and differences between GR and MR signaling in the presence of glucocorticoid hormones, we generated U-2 OS (human osteosarcoma) cell lines stably expressing GR, MR, and both GR and MR (MRGR). Immunofluorescence analysis showed that the treatment of these cell lines with 1 nM of the synthetic glucocorticoid dexamethasone (Dex) induced nuclear translocation of both GR and MR. Moreover, Proximity Ligation Assay revealed that, in the absence of ligand, GR associated with MR in the cytoplasm and, upon 1 nM Dex exposure, GR-MR complexes were detected in the nucleus of MRGR cells. To decipher the functional contribution of GR-MR complexes in the transcriptional response to Dex, we performed RNA-seq in GR, MR, and MRGR cells treated with 1 nM of Dex. Transcriptome analysis revealed that Dex-activated GR regulated the transcription of 6180 genes. Co-expression of MR resulted in a greatly blunted Dex-mediated gene response which reduced the glucocorticoid-dependent transcriptome size by 75%. This phenomenon was also observed using a higher concentration of Dex. Indeed, 40% of genes commonly regulated by Dex in GR and MRGR cells showed a reduced magnitude of regulation when MR is co-expressed. These results suggest a functional antagonism between GR and MR in which MR inhibits GR function. Understanding the molecular mechanisms governing the cross-talk between GR and MR is crucial for the development of new therapies that address the adverse effects of glucocorticoid treatment as well as for the discovery of novel glucocorticoid-based therapeutics with minimal side effects.