The Human Type 2 Iodothyronine Deiodinase Is a Selenoprotein Highly Expressed in a Mesothelioma Cell Line

Types 1 and 3 iodothyronine deiodinases are known to be selenocysteine-containing enzymes. Although a putative human type 2 iodothyronine deiodinase (D2) gene (hDio2) encoding a similar selenoprotein has been identified, basal D2 activity is not selenium (Se)-dependent nor has D2 been labeled with ⁷⁵Se. A human mesothelioma cell line (MSTO-211H) has recently been shown to have ∼40-fold higher levels of hDio2 mRNA than mesothelial cells. Mesothelioma cell lysates activate thyroxine (T₄) to 3,5,3′-triiodothyronine with typical characteristics of D2 such as low K_m (T₄), 1.3 nM, resistance to propylthiouracil, and a short half-life (∼30 min). D2 activity is ∼30-fold higher in Se-supplemented than in Se-depleted medium. An antiserum prepared against a peptide deduced from the Dio2 mRNA sequence precipitates a ⁷⁵Se protein of the predicted 31-kDa size from ⁷⁵Se-labeled mesothelioma cells. Bromoadenosine 3′5′ cyclic monophosphate increases D2 activity and ⁷⁵Se-p31 ∼2.5-fold whereas substrate (T ₄) reduces both D2 activity and ⁷⁵Se-p31 ∼2-3-fold. MG132 or lactacystin (10 μM), inhibitors of the proteasome pathway by which D2 is degraded, increase both D2 activity and ⁷⁵Se-p31 3-4-fold and prevent the loss of D2 activity during cycloheximide or substrate (T ₄) exposure. Immunocytochemical studies with affinity-purified anti-hD2 antibody show a Se-dependent increase in immunofluorescence. Thus, human D2 is encoded by hDio2 and is a member of the selenodeiodinase family accounting for its highly catalytic efficiency in T₄ activation.