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.
Diederich
S
S
Department of Endocrinology, Klinikum Benjamin Franklin, Freie Universität Berlin, Germany.
Grossmann
C
C
Hanke
B
B
Quinkler
M
M
Herrmann
M
M
Bähr
V
V
Oelkers
W
W
D042842:11-beta-Hydroxysteroid Dehydrogenases; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002635:Chenodeoxycholic Acid / Q000494:pharmacology*; D003907:Dexamethasone / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D006913:Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors*; D007527:Isoenzymes / Q000037:antagonists & inhibitors*; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D008875:Middle Aged; D011725:Pyridines / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007527:Isoenzymes; D011725:Pyridines; D002635:Chenodeoxycholic Acid; C013109:metapyrone; D003907:Dexamethasone; D006913:Hydroxysteroid Dehydrogenases; D042842:11-beta-Hydroxysteroid Dehydrogenases