A second enzyme protecting mineralocorticoid receptors from glucocorticoid occupancy

David J. Morris, Syed A. Latif, Michael D. Rokaw, Charles O. Watlington, John P. Johnson
1998 American Journal of Physiology - Cell Physiology  
A second enzyme protecting mineralocorticoid receptors from glucocorticoid occupancy. Am. J. Physiol. 274 (Cell Physiol. 43): C1245-C1252, 1998.-We have confirmed that A6 cells (derived from kidney of Xenopus laevis), which contain both mineralocorticoid and glucocorticoid receptors, do not normally possess 11␤-hydroxysteroid dehydroxgenase (11␤-HSD1 or 11␤-HSD2) enzymatic activity and so are without apparent "protective" enzymes. A6 cells do not convert the glucocorticoid corticosterone to
more » ... ehydrocorticosterone but do, however, possess steroid 6␤-hydroxylase that transforms corticosterone to 6␤-hydroxycorticosterone. This hydroxylase is cytochrome P-450 3A (CYP3A). We have now determined the effects of 3␣,5␤-tetrahydroprogesterone and chenodeoxycholic acid (both inhibitors of 11␤-HSD1) and 11-dehydrocorticosterone and 11␤-hydroxy-3␣,5␤-tetrahydroprogesterone (inhibitors of 11␤-HSD2) and carbenoxalone, which inhibits both 11␤-HSD1 and 11␤-HSD2, on the actions and metabolism of corticosterone and active Na ϩ transport [short-circuit current (I sc )] in A6 cells. All of these 11␤-HSD inhibitory substances induced a significant increment in corticosterone-induced I sc , which was detectable within 2 h. However, none of these agents caused an increase in I sc when incubated by themselves with A6 cells. In all cases, the additional I sc was inhibited by the mineralocorticoid receptor (MR) antagonist, RU-28318, whereas the original I sc elicited by corticosterone alone was inhibited by the glucocorticoid receptor antagonist, RU-38486. In separate experiments, each agent was shown to significantly inhibit metabolism of corticosterone to 6␤hydroxycorticosterone in A6 cells, and a linear relationship existed between 6␤-hydroxylase inhibition and the MRmediated increase in I sc in the one inhibitor tested. Troleandomycin, a selective inhibitor of CYP3A, inhibited 6␤-hydroxylase and also significantly enhanced corticosterone-induced I sc at 2 h. These experiments indicate that the enhanced MR-mediated I sc in A6 cells may be related to inhibition of 6␤-hydroxylase activity in these cells and that this 6␤hydroxylase (CYP3A) may be protecting the expression of corticosterone-induced active Na ϩ transport in A6 cells by MR-mediated mechanism(s). steroid 6␤-hydroxylase; sodium transport 0363-6143/98 $5.00
doi:10.1152/ajpcell.1998.274.5.c1245 fatcat:z34whji22zg5laqxjlpx2lsg3e