5α-reduced glucocorticoids, novel endogenous activators of the glucocorticoid receptor

Metabolism of glucocorticoids to A-ring-reduced dihydro- and tetrahydro-derivatives by means of hepatic 5α- and 5β-reductases has long been regarded as a pathway of irreversible inactivation. However, 5α-reduced metabolites of other steroids, e.g. testosterone and aldosterone, have significant biological activity. We investigated whether 5α-reduced metabolites of corticosterone are glucocorticoid receptor (GR) agonists. Corticosterone, 5α-tetrahydrocorticosterone (5αTHB), and 5α-dihydrocorticosterone (5αDHB) were similarly effective in displacing tritiated dexamethasone from binding sites in hepatocytes, whereas 5β-reduced metabolites were less effective in binding. 5αTHB had glucocorticoid receptor agonist effects in vitro and in vivo. After transient co-transfection of hGR and a murine mammary tumor virus-luciferase reporter into HeLa cells, 5αTHB was active to a comparable extent as corticosterone (28-fold versus 37-fold induction, respectively, at 1 μM) and additive to the effect of corticosterone. 5β-Reduced metabolites did not activate GR. In H4IIE hepatoma cells, both 5αTHB and corticosterone induced mRNA expression of tyrosine aminotransferase and phosphoenolpyruvate carboxykinase (6.0- versus 10.1-fold and 3.5- versus 3.9-fold at 1 μM, respectively), an effect that was inhibited by RU486. To assess in vivo glucocorticoid activity, suppression of plasma ACTH was demonstrated in adrenalectomized rats after intraperitoneal administration of vehicle (ACTH trough 80.2 pM), corticosterone (5 mg/kg; 22 pM, p < 0.001) or 5αTHB (5 mg/kg; 51.3 pM, p < 0.005). Similar endogenous concentrations of corticosterone and 5αTHB were detected in rat liver homogenates by gas chromatography mass spectrometry. We conclude that 5α-reduced glucocorticoids bind to and activate GR. Transcription of glucocorticoid-regulated genes in tissues that express 5α-reductases will thus be influenced by intracellular levels of both corticosterone and its 5α-reduced metabolites.