Renal transplant recipients treated with cyclosporine A (CsA) and FK506 (tacrolimus) develop signs of hypoaldosteronism despite normal plasma aldosterone levels, suggesting a relative resistance of the distal nephron to aldosterone action. To examine the effects of immunosuppressants on human MR (hMR) function, we established the M cell model, renal tubular cells stably transfected with hMR. Upon CsA and FK506 administration, hMR mRNA levels and aldosterone binding in M cells remained unchanged

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... (maximum number of sites, ϳ80 fmol/mg protein; K d ‫؍‬ ϳ1 nM). Aldosterone-dependent intracellular localization of green fluorescent protein-hMR was not affected by immunosuppressants. A major impact of CsA or FK506 on the multidrug resistance gene product in cellular accumulation of aldosterone was also excluded. In contrast, aldosterone-stimulated hMR transcriptional activity was reduced to 53 ؎ 11.2% (P < 0.03) after pretreatment of M cells for 3 d with CsA and to 71 ؎ 9.6% (P < 0.05) after pretreatment with FK506. These effects were both time and concentration dependent (IC 50 of CsA, 10 ؊6 M; IC 50 of FK506, 10 ؊5 M) and needed at least 2 d to develop. Such an inhibitory effect does not depend on the N-terminal part of hMR, as CsA also reduced transcriptional activity of a 1-453 deletion mutant of hMR. Our results demonstrate that immunosuppressants inhibit hMR transcriptional activity without affecting hMR expression, aldosterone binding properties, and hMR nucleocytoplasmic trafficking. They suggest that ion transport alterations in renal graft recipients are in part induced by impaired hMR function. (Endocrinology 143: 1932-1941, 2002) Abbreviations: AF, Activating function; CsA, cyclosporine A; EGFP, enhanced green fluorescent protein; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; hMR, human MR; hsp90, heat shock protein 90; MDR, multidrug resistance gene product; MMTV, mouse mammary tumor virus; P-gp, P-glycoprotein.