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

Objective: Selective inhibitors of 11 beta-hydroxysteroid-dehydrogenase typ e I may be of therapeutical interest for two reasons: i) 9 alpha-Fluorinate d 11-dehydrosteroids like 11-dehydro-dexamethasone (DH-D) are rapidly activ ated by human kidney 11 beta-hydroxysteroid-dehydrogenase type II (11 beta- HSD-II) to dexamethasone (D), If the same reaction by hepatic 11 beta-HSD-I could be selectively inhibited, DH-D could be used for selective renal imm unosuppressive therapy. ii) Reduction of cortisone to cortisol in the liver may increase insulin resistance in type 2 diabetes mellitus, and inhibitio n. of the enzyme may lead to a decrease in gluconeogenesis, Therefore, we characterized the metabolism of DPI-D by human hepatic 11 bet a-HSD-I and tried to find a selective inhibitor of this isoenzyme. Methods: For kinetic analysis of 11 beta-HSD-I, we used microsomes prepared from unaffected parts of liver segments, resected be cause of hepatocarcin oma or metastatic disease, For inhibition experiments, we also tested 11 be ta-HSD-II activity with human kidney cortex microsomes, The inhibitory pote ncy of several compounds was evaluated for oxidation and reduction in conce ntrations from 10(-9) to 10(-5) mol/l. Results: Whereas D was not oxidized by human liver microsomes at all, corti sol was oxidized to cortisone with a maximum velocity (V-max) of 95 pmol/mg per min. The reduction of DH-D to D (V-max = 742 pmol/mg per min, Michaeli s-Menten constant (K-m) = 1.6 mu mol/l) was faster than that of cortisone t o cortisol (V-max = 187 pmol/mg per min). All reactions tested in liner mic rosomes showed the characteristics of 11 beta-HSD-I: K-m values in the micr omolar range, preferred cosubstrate NADP(W), no product inhibition. Of the substances tested for inhibition of 11 beta-HSD-I and -II, chenodeoxycholic acid was the only one that selectively inhibited 11 beta-HSD-I (IC50 for r eduction: 2.8 x 10(-6) mol/l, IC50 for oxidation: 4.4 x 10(-6) mol/l). wher eas ketoconazole preferentially inhibited oxidation and reduction reactions catalyzed by 11 beta-HSD-II. Metyrapone, which is reduced to metyrapol by hepatic 11 beta-HSD-I, inhibited steroid reductase activity of 11 beta-HSD- I and -II and oxidative activity of 11 beta-HSD-II. These findings can be e xplained by substrate competition for reductase reactions and by product in hibition of the oxidation, which is a well-known characteristic of 11 beta- HSD-II. Conclusions: Our in vitro results may offer a new concept for renal glucoco rticoid targeting. Oral administration of small amounts of DH-D (low substr ate affinity for 11 beta-HSD-I) in combination with chenodeoxycholic acid ( selective inhibition of 11 beta-HSD-I) may prevent hepatic first pass reduc tion of DH-D, thus allowing selective activation of DH-D to D by the high a ffinity 11 beta-HSD-II in the kidney. Moreover, selective inhibitors of the hepatic 11 beta-HSD-I, like chenodeoxycholic acid, may become useful in th e therapy of patients with hepatic insulin resistance including diabetes me llitus type II, because cortisol enhances gluconeogenesis.