NAD-dependent 11β-hydroxysteroid dehydrogenase in cultured human colonic epithelial cells

The inactivation of physiological glucocorticoids by 11β-hydroxysteroid dehydrogenase (11β-HSD) confers mineralocorticoid specificity to certain aldosterone target tissues. However, 11β-HSD activity in a human mineralocorticoid-responsive tissue has never been characterized. The present studies describe the features of 11β-HSD in the cultured human colonic epithelial cell line, T84. The 11β-HSD activity of T84 cells resided in the microsomal fraction and showed a marked preference for NAD rather than NADP as cofactor. NAD or NADP (200 μM) increased the conversion of corticosterone to 11-dehydrocorticosterone by 24.1 ± 2.1 and 0.5 ± 0.7 pmol · mg protein^-1 · 20 min^-1, respectively, indicating a > 40-fold preference for NAD vs. NADP. The Michaelis constant values for corticosterone and cortisol were 11.3 ± 1.5 and 79.8 ± 10 nM, respectively. The T84 11β-HSD was inhibited by 11-dehydrocorticosterone in a noncompetitive fashion [inhibition constant (K(i)) = 180 ± 9.6 nM] and by carbenoxolone in a competetive fashion (K(i) = 17.4 ± 1.3 nM). The expression of mineralocorticoid receptors in these cells was demonstrated by reverse transcriptase-polymerase chain reaction of mRNA isolated from T84 cells and by [³H] aldosterone binding studies. The coexpression of this NAD-dependent isoform of 11β-HSD and mineralocorticoid receptors is consistent with the view that the NAD-dependent isoform is responsible for the specificity of mineralocorticoid responses.