In the search for specific inhibitors of human 11beta-hydroxysteroid-dehydrogenases (11beta-HSDs): chenodeoxycholic acid selectively inhibits 11beta-HSD-I.

OBJECTIVE: Selective inhibitors of 11beta-hydroxysteroid-dehydrogenase type I may be of therapeutical interest for two reasons: i) 9alpha-Fluorinated 11-dehydrosteroids like 11-dehydro-dexamethasone (DH-D) are rapidly activated by human kidney 11beta-hydroxysteroid-dehydrogenase type II (11beta-HSD-II) to dexamethasone (D). If the same reaction by hepatic 11beta-HSD-I could be selectively inhibited, DH-D could be used for selective renal immunosuppressive therapy. ii) Reduction of cortisone to cortisol in the liver may increase insulin resistance in type 2 diabetes mellitus, and inhibition of the enzyme may lead to a decrease in gluconeogenesis. Therefore, we characterized the metabolism of DH-D by human hepatic 11beta-HSD-I and tried to find a selective inhibitor of this isoenzyme.
METHODS: For kinetic analysis of 11beta-HSD-I, we used microsomes prepared from unaffected parts of liver segments, resected because of hepatocarcinoma or metastatic disease. For inhibition experiments, we also tested 11beta-HSD-II activity with human kidney cortex microsomes. The inhibitory potency of several compounds was evaluated for oxidation and reduction in concentrations from 10(-9) to 10(-5)mol/l.
RESULTS: Whereas D was not oxidized by human liver microsomes at all, cortisol was oxidized to cortisone with a maximum velocity (V(max)) of 95pmol/mg per min. The reduction of DH-D to D (V(max)=742pmol/mg per min, Michaelis--Menten constant (K(m))=1.6 micromol/l) was faster than that of cortisone to cortisol (V(max)=187pmol/mg per min). All reactions tested in liver microsomes showed the characteristics of 11beta-HSD-I: K(m) values in the micromolar range, preferred cosubstrate NADP(H), no product inhibition. Of the substances tested for inhibition of 11beta-HSD-I and -II, chenodeoxycholic acid was the only one that selectively inhibited 11beta-HSD-I (IC(50) for reduction: 2.8x10(-6)mol/l, IC(50) for oxidation: 4.4x10(-6)mol/l), whereas ketoconazole preferentially inhibited oxidation and reduction reactions catalyzed by 11beta-HSD-II. Metyrapone, which is reduced to metyrapol by hepatic 11beta-HSD-I, inhibited steroid reductase activity of 11beta-HSD-I and -II and oxidative activity of 11beta-HSD-II. These findings can be explained by substrate competition for reductase reactions and by product inhibition of the oxidation, which is a well-known characteristic of 11beta-HSD-II.
CONCLUSIONS: Our in vitro results may offer a new concept for renal glucocorticoid targeting. Oral administration of small amounts of DH-D (low substrate affinity for 11beta-HSD-I) in combination with chenodeoxycholic acid (selective inhibition of 11beta-HSD-I) may prevent hepatic first pass reduction of DH-D, thus allowing selective activation of DH-D to D by the high affinity 11beta-HSD-II in the kidney. Moreover, selective inhibitors of the hepatic 11beta-HSD-I, like chenodeoxycholic acid, may become useful in the therapy of patients with hepatic insulin resistance including diabetes mellitus type II, because cortisol enhances gluconeogenesis.