Hexose-6-phosphate Dehydrogenase Modulates 11β-Hydroxysteroid Dehydrogenase Type 1-Dependent Metabolism of 7-keto- and 7β-hydroxy-neurosteroids

Background. The role of 11 beta-hydroxysteroid dehydrogenase type 1 (11 beta-HSD1) in the regulation of energy metabolism and immune system by locally reactivating glucocorticoids has been extensively studied. Experiments determining initial rates of enzyme activity revealed that 11 beta-HSD1 can catalyze both the reductase and the dehydrogenase reaction in cell lysates, whereas it predominantly catalyzes the reduction of cortisone to cortisol in intact cells that also express hexose-6-phosphate dehydrogenase (H6PDH), which provides cofactor NADPH. Besides its role in glucocorticoid metabolism, there is evidence that 11 beta-HSD1 is involved in the metabolism of 7-keto- and 7-hydroxy-steroids; however the impact of H6PDH on this alternative function of 11 beta-HSD1 has not been assessed. Methodology. We investigated the 11 beta-HSD1-dependent metabolism of the neurosteroids 7-keto-, 7 alpha-hydroxy- and 7 beta-hydroxy-dehydroepiandrosterone ( DHEA) and 7-keto- and 7 beta-hydroxy-pregnenolone, respectively, in the absence or presence of H6PDH in intact cells. 3D-structural modeling was applied to study the binding of ligands in 11 beta-HSD1. Principal Findings. We demonstrated that 11 beta-HSD1 functions in a reversible way and efficiently catalyzed the interconversion of these 7-keto- and 7-hydroxy-neurosteroids in intact cells. In the presence of H6PDH, 11 beta-HSD1 predominantly converted 7-keto-DHEA and 7-ketopregnenolone into their corresponding 7 beta-hydroxy metabolites, indicating a role for H6PDH and 11 beta-HSD1 in the local generation of 7 beta-hydroxy-neurosteroids. 3D-structural modeling offered an explanation for the preferred formation of 7-hydroxy-neurosteroids. Conclusions. Our results from experiments determining the steady state concentrations of glucocorticoids or 7-oxygenated neurosteroids suggested that the equilibrium between cortisone and cortisol and between 7-keto- and 7-hydroxy-neurosteroids is regulated by 11 beta-HSD1 and greatly depends on the coexpression with H6PDH. Thus, the impact of H6PDH on 11 beta-HSD1 activity has to be considered for understanding both glucocorticoid and neurosteroid action in different tissues.