title: Calcium inhibits human placental 11beta-hydroxysteroid dehydrogenase type 2 activity.
abstract: The effect of Ca2+ on the conversion of cortisol to its inert metabolite cortisone, the reaction catalyzed by the microsomal enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), was investigated in human placental microsomes. Placental microsomal 11beta-HSD2 activity, as determined by the rate of conversion of cortisol to cortisone, was inhibited up to 50% by increasing free Ca2+ concentrations from 22 to 268 nM. The Ca2+-induced inhibition was reversible since chelation of endogenous Ca2+ with EGTA increased 11beta-HSD2 activity up to 200%. Ca2+ decreased the maximal velocity (Vmax) of the 11beta-HSD2 catalyzed conversion of cortisol to cortisone without altering the Km of 11beta-HSD2 for cortisol, indicating that Ca2+ modulates the catalytic efficiency rather than the substrate binding of 11beta-HSD2. Moreover, the Ca2+-induced inhibition does not appear to involve altered cofactor (NAD+) binding since the inhibition of microsomal 11beta-HSD2 activity by a sub-maximal concentration of free Ca2+ was not overcome by increasing the concentration of NAD+. These findings in the microsomes were then extended to an intact cell system, JEG-3 cells, an established model for human placental trophoblasts. In these cells, an increase in cytosolic free Ca2+ concentration ([Ca2+]i) elicited by a known physiological stimulus, PGF(2alpha), was accompanied by a 40% decrease in the level of 11beta-HSD2 activity. Furthermore, the PGF(2alpha)-induced inhibition of 11beta-HSD2 activity was abrogated when increases in [Ca2+]i were blocked with the intracellular Ca2+ chelator, BAPTA. Collectively, these results demonstrate for the first time that Ca2+ inhibits human placental 11beta-HSD2 activity by a post-translational mechanism not involving substrate or cofactor binding.
authors.affiliation: CIHR Group in Fetal and Neonatal Health and Development, Lawson Health Research Institute, St. Joseph's Health Care of London, Department of Obstetrics and Gynaecology, University of Western Ontario, 268 Grosvenor Street, London, Ontario, Canada.
authors.forename: D B
authors.initials: DB
authors.lastname: Hardy
authors.forename: S J
authors.initials: SJ
authors.lastname: Dixon
authors.forename: N
authors.initials: N
authors.lastname: Narayanan
authors.forename: K
authors.initials: K
authors.lastname: Yang
mesh_terms: D043209:11-beta-Hydroxysteroid Dehydrogenase Type 2; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D006801:Humans; D006913:Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D010920:Placenta / Q000201:enzymology*; D011499:Protein Processing, Post-Translational
chemical_list: D006913:Hydroxysteroid Dehydrogenases; D043209:11-beta-Hydroxysteroid Dehydrogenase Type 2; C468387:HSD11B2 protein, human; D002118:Calcium