Calcium Inhibits Human Placental 11β-Hydroxysteroid Dehydrogenase Type 2 Activity

The effect of Ca2+ on the conversion of cortisol to its inert metabolite cortisone, the reaction catalyzed by the microsomal enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), was investigated in human placental microsomes. Placental microsomal 11β-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 11β-HSD2 activity up to 200%. Ca2+ decreased the maximal velocity (Vmax) of the 11β-HSD2 catalyzed conversion of cortisol to cortisone without altering the Km of 11β-HSD2 for cortisol, indicating that Ca2+ modulates the catalytic efficiency rather than the substrate binding of 11β-HSD2. Moreover, the Ca2+-induced inhibition does not appear to involve altered cofactor (NAD+) binding since the inhibition of microsomal 11β-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, PGF2α, was accompanied by a 40% decrease in the level of 11β-HSD2 activity. Furthermore, the PGF2α-induced inhibition of 11β-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 11β-HSD2 activity by a post-translational mechanism not involving substrate or cofactor binding.