5 alpha -Reduced Glucocorticoids, Novel Endogenous Activators of the Glucocorticoid Receptor

• Published in: The Journal of biological chemistry 2004-05, Vol.279 (22), p.22908-22912
• Main Authors: McInnes, K J, Kenyon, C J, Chapman, KE, Livingstone, DEW, Macdonald, L J, Walker, B R, Andrew, R
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M402822200

Metabolism of glucocorticoids to A-ring-reduced dihydro- and tetrahydro- derivatives by means of hepatic 5 alpha - and 5 beta -reductases has long been regarded as a pathway of irreversible inactivation. However, 5 alpha -reduced metabolites of other steroids, e.g. testosterone and aldosterone, have significant biological activity. We investigated whether 5 alpha -reduced metabolites of corticosterone are glucocorticoid receptor (GR) agonists. Corticosterone, 5 alpha -tetrahydrocorticosterone (5 alpha THB), and 5 alpha - dihydrocorticosterone (5 alpha DHB) were similarly effective in displacing tritiated dexamethasone from binding sites in hepatocytes, whereas 5 beta - reduced metabolites were less effective in binding. 5 alpha THB 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, 5 alpha THB was active to a comparable extent as corticosterone (28- fold versus 37-fold induction, respectively, at 1 mu M) and additive to the effect of corticosterone. 5 beta -Reduced metabolites did not activate GR. In H4IIE hepatoma cells, both 5 alpha THB 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 mu M, 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 5 alpha THB (5 mg/kg; 51.3 pM, p < 0.005). Similar endogenous concentrations of corticosterone and 5 alpha THB were detected in rat liver homogenates by gas chromatography mass spectrometry. We conclude that 5 alpha -reduced glucocorticoids bind to and activate GR. Transcription of glucocorticoid-regulated genes in tissues that express 5 alpha -reductases will thus be influenced by intracellular levels of both corticosterone and its 5 alpha -reduced metabolites.