11β-Hydroxysteroid dehydrogenase in the rat adrenal

ABSTRACT

11β-Hydroxysteroid dehydrogenase (11β-HSD) catalyses the interconversion of biologically active cortisol to inactive cortisone in man, and corticosterone to 11-dehydrocorticosterone in rodents. As such, this enzyme has been shown to confer aldosterone-selectivity on the mineralocorticoid receptor and to modulate cortisol/corticosterone access to the glucocorticoid receptor (GR). Two kinetically distinct isoforms of this enzyme have been characterized in both rodents and man; a low-affinity NADP(H)-dependent enzyme (11β-HSD1) which predominantly acts as an oxo-reductase and, more recently, a high-affinity NAD-dependent uni-directional dehydrogenase (11β-HSD2). In this study we have analysed the expression of both 11β-HSD1 and 11β-HSD2 isoforms in rat adrenal cortex and medulla and have investigated their possible roles with respect to glucocorticoid-regulated enzymes mediating catecholamine biosynthesis in adrenal medullary chromaffin cells.

Using a rat 11β-HSD1 probe and a recently cloned in-house mouse 11β-HSD2 cDNA probe, Northern blot analyses revealed expression of mRNA species encoding both 11β-HSD1 (1·4kb) and 11β-HSD2 (1·9kb) in the whole adrenal. Consistent with this, 11β-dehydrogenase activity (pmol 11-dehydrocorticosterone formed/mg protein per h, mean ± s.e.m.) in adrenal homogenates, when incubated with 50 nm corticosterone in the presence of 200 μm NAD, was 97·0 ± 9·0 and with 500 nm corticosterone in the presence of 200 μm NADP, was 98·0 ± 1·4 11-Oxoreductase activity (pmol corticosterone formed/mg protein per h) with 500 nm 11-dehydrocorticosterone in the presence of 200 μm NADPH, was 187·7 ± 31·2. In situ hybridization studies of rat adrenal cortex and medulla using ³⁵S-labelled antisense 11β-HSD1 cRNA probe revealed specific localization of 11β-HSD1 mRNA expression predominantly to cells at the corticomedullary junction, most likely within the inner cortex. In contrast, 11β-HSD2 mRNA was more abundant in cortex versus medulla, and was more uniformly distributed over the adrenal gland. Negligible staining was detected using control sense probes.

Ingestion of the 11β-HSD inhibitor, glycyrrhizic acid (>100mg/kg body weight per day for 4 days) resulted in significant inhibition of adrenal NADP-dependent (98·0 ± 1·4 vs 42·5 ± 0·4) and NAD-dependent (97·0 ± 9·0 vs 73·2 ± 6·7) 11β-dehydrogenase activity and 11-oxoreductase activity (187·7 ± 31·2 vs 67·7 ± 15·3). However, while levels of 11β-HSD1 mRNA were similarly reduced (0·85 ± 0·07 vs 0·50 ± 0·05 arbitrary units), those for 11β-HSD2 remained unchanged (0·44 ± 0·03 vs 0·38 ± 0·01). Levels of mRNA encoding the glucocorticoid-dependent enzyme phenylethanolamine N-methyltransferase which catalyses the conversion of noradrenaline to adrenaline, were also significantly reduced in those rats given glycyrrhizic acid (1·12 ± 0·04 vs 0·78 ± 0·04), while those for the glucocorticoid-independent enzyme tyrosine hydroxylase (1·9 kb), which catalyses the conversion of tyrosine to DOPA, were unchanged (0·64 ± 0·04 vs 0·61 ± 0·04).

In conclusion, the rat adrenal gland expresses both 11β-HSD1 and 11β-HSD2 isoforms. 11β-HSDl gene expression is localized to the adrenal cortico-medullary junction, where it is ideally placed to regulate the supply of cortex-derived corticosterone to the medullary chromaffin cells. This, together with our in vivo studies, suggests that 11β-HSD1 may play an important role with respect to adrenocorticosteroid regulation of adrenaline biosynthesis. The role of 11β-HSD2 in the adrenal remains to be elucidated.