Lay summary

Glucocorticoids are involved in the regulation of most physiological processes and impaired glucocorticoid action has been associated with several major diseases. The aim of this project was to elucidate molecular mechanisms underlying the control of local glucocorticoid availability and activation of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in different situations. Further, since local glucocorticoid activation depends on cofactor NADPH in the endoplasmic reticulum (ER), we studied mechanisms affecting luminal NADPH availability. Biochemical and cell biological methods were applied. Using 3D-structure modeling and biochemical methods, we attempted to identify inhibitors of 11beta-hydroxysteroid dehydrogenase (11b-HSD) 1 and 2, which convert inactive into active glucocorticoids and the reverse reaction, respectively.

Our results provide evidence for the existence of a hexose-6-phosphate isomerase in the ER of hepatocytes and adipocytes. This isomerase may explain the observed stimulation of NADPH dependent glucocorticoid activation by fructose-6-phosphate and glucose-6-phospate. High fructose diet, by increasing levels of active glucocorticoids, may contribute to metabolic disturbances. Using hepatocyte cell models, we found that glucocorticoids suppress the transcription factor Nrf2, which is involved in cellular defense against oxidative stress. Thus, chronically elevated glucocorticoids may cause impaired defense against oxidative stress. Moreover, we also discovered a new function of 11b-HSD1 in the oxoreduction of the bile acid 7-oxolithocholic acid. Elevated glucocorticoids may thus interfere with bile acid metabolism, which may be relevant for detoxification of oxidized lipids and xenobiotics. Employing a macrophage/microglia cell model, we demonstrated a differential regulation of the NFkB pathway and the control of inflammatory mediators by the GR and MR. We showed that MR antagonists and 11b-HSD1 inhibitors effectively modulated the inflammatory parameters. The balanced action of the two receptors was tightly regulated by 11b-HSD1-dependent glucocorticoid activation. Finally, we identified several glycyrrhetinic acid-derived compounds that selectively inhibited 11b-HSD1 and 2, respectively, that can be used in future in vitro and in vivo studies to modulate glucocorticoid availability.

The results provide a novel link between NADPH generation and utilization in the ER and the control of glucocorticoid formation. The results impact on local activation of GR and MR, which is important in the regulation of metabolism and immune responses. The results are of interest for basic researchers, clinical scientists and drug development specialists.