T(4) activation into T(3) is catalyzed by
type 2 deiodinase (D2) in the brain. The rapid induction of D2 in astrocytes by transient
brain ischemia has prompted us to explore the effects of
hypoxia on D2 in cultures of astrocytes.
Hypoxia (2.5% O(2)) of cultured astrocytes increased D2 activity, alone or in association with agents stimulating the cAMP pathway.
Hypoxia had no effect on D2
mRNA accumulation.
Cycloheximide did not block the effect of
hypoxia on D2 activity and D2 half-life was enhanced under
hypoxia demonstrating a posttranslational action of
hypoxia. Furthermore, the D2 activity increase by
hypoxia was not additive with the increase promoted by the
proteasome inhibitor carbobenzoxy-L-leucyl-L-leucyl-
L-leucinal (
MG132). This strongly suggests that
hypoxia leads to stabilization of D2 by slowing its degradation by the
proteasome pathway.
Hypoxia, in contrast to
MG132, did not block the T(4)-induced D2 inactivation. A contribution of
prolyl hydroxylase to the
hypoxia effects on D2 was also suggested on the basis of increased D2 activity after addition of different
prolyl hydroxylase inhibitors (
cobalt chloride,
desferrioxamine,
dimethyloxalylglycine, dimethylsuccinate). Specific inhibitors of ERK,
p38 MAPK, or
phosphatidylinositol 3-kinase pathways were without any effect on
hypoxia-increased D2 activity, eliminating their role in the effects of
hypoxia. Interestingly,
diphenyleneiodonium, an inhibitor of
nicotinamide adenine dinucleotide phosphate oxidase inhibited the
hypoxia-increased D2 indicating a role for some
reactive oxygen species in the mechanism of D2 increase. Further studies are required to clarify the precise molecular mechanisms involved in the D2 stabilization by
hypoxia.