Background: Dexamethasone (DEX) can cause severe neuropsychiatric adverse effects. As DEX is a glucocorticoid receptor (GR) agonist, it suppresses the production of cortisol and thereby depletes the mineralocorticoid receptor (MR) of its ligand, rendering MR inactive. Next to the strongly stimulated GR, the reduced MR activity by DEX might be the cause of the neuropsychiatric effects. Currently, we are performing an RCT to investigate the MR refill concept: adding hydrocortisone to DEX to restore MR activity and thereby reducing the adverse side effects of DEX. In addition, we try to unravel the underlying mechanism on a cellular level.

Objective: We aim to validate the MR refill concept by evaluation of MR activity in the brain of a pediatric subject treated with DEX, and in transfected HEK293T cells. We determined both MR nuclear translocation and MR-dependent gene expression.

Methods: A luciferase reporter assay was performed on MR transfected HEK293T cells. The cells were stimulated with DEX for 24 hours and MR activity was measured. Live cell imaging was performed to visualize translocation of MR in these cells by stimulation of DEX for 30 minutes. Functionality of MR was assessed with q-PCR for target genes on DEX-treated tissue and controls.

Results: DEX showed activation of MR, but not as strong as the natural MR ligands aldosterone and cortisol. Of note, higher concentrations of DEX were needed for MR activation. Live cell imaging visualized cytoplasmic MR staining with lower concentrations of DEX and nuclear MR was observed with higher concentrations. Expression of putative MR target genes was low compared to unaffected controls.

Conclusion: Overall, DEX was able to activate MR in human cells with low potency and low efficacy. We observed nuclear translocation of MR in cells treated with higher DEX concentrations. Furthermore, low MR target gene expression was observed in the DEX treated tissue. These findings suggest that high dosages of DEX could activate MR to an extent that allows nuclear translocation, without inducing expression of MR target genes.