Corticosterone induces 11 beta-HSD and mineralocorticoid specificity in an amphibian urinary bladder cell line.

We have examined the mineralocorticoid specificity in a TBM 18-23 cell line derived from the toad bladder epithelium. In cells grown on porous substrate, corticosterone was more potent than aldosterone in stimulating a sodium transport response, measured by the short-circuit current method 6 h after hormone addition [mean affinity constant (K0.5) for corticosterone = 1 nM vs. K0.5 for aldosterone = 8 nM]. The time course of effects and saturation kinetics were identical for both agonists, suggesting interaction with identical receptors. Whereas the dose-response relationship for aldosterone did not change with time of incubation (6 vs. 24 h), the dose-response curve for corticosterone became biphasic at 24-h incubation (apparent K0.5 as high as 40 nM), demonstrating that corticosterone became apparently less potent with time. Pretreatment with carbenoxolone, a potent inhibitor of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD), restored full sensitivity at 24-h incubation to corticosterone. The 11 beta-HSD activity was low during the first 3 h of incubation in the presence of 3 nM corticosterone, and only a small fraction (approximately 7%) of corticosterone was metabolized. At 24-h incubation, 11 beta-HSD activity increased approximately 2.5-fold (P < 0.001, n = 8). We conclude that 11 beta-HSD activity is induced by its own substrate in TBM cells in parallel with the induction of the carbenoxolone-sensitive sodium transport response.

Rat liver 11 beta-hydroxysteroid dehydrogenase complementary deoxyribonucleic acid encodes oxoreductase activity in a mineralocorticoid-responsive toad bladder cell line.

The mineralocorticoid receptor displays equal affinity for aldosterone and corticosterone. It has been proposed that aldosterone selectivity in vivo is achieved by the conversion of corticosterone into its inactive metabolite 11-dehydrocorticosterone by 11 beta-hydroxysteroid dehydrogenase (11 beta HSD). To test this hypothesis, we transfected rat liver 11 beta HSD cDNA into TBM cells, a sodium-transporting cell line. These cells respond equally well to aldosterone and corticosterone, indicating that endogenous 11 beta HSD is expressed at low levels in TBM cells. Although exogenous rat liver 11 beta HSD was expressed at high levels in transfected cells, mineralocorticoid selectivity was not observed. By contrast, the biologically inactive 11-dehydrocorticosterone was readily converted into corticosterone, a potent agonist for sodium transport. Our results indicate that rat liver 11 beta HSD behaves predominantly as a reductase in TBM cells. Another 11 beta HSD isoform is likely to be responsible for the dehydrogenase reaction in aldosterone-responsive cells.