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-cont aining enzymes. Although a putative human type 2 iodothyronine deiodinase ( D2) gene (hDio2) encoding a similar selenoprotein has been identified, basa l D2 activity is not selenium (Se)-dependent nor has D2 been labeled with S e-75. A human mesothelioma cell line (MSTO-211H) has recently been shown to have similar to 40-fold higher levels of hDio2 mRNA than mesothelial cells . Mesothelioma cell lysates activate thyroxine (T-4) to 3,5,3 ' -triiodothy ronine with typical characteristics of D2 such as low K-m (T-4), 1.3 mi, re sistance to propylthiouracil, and a short half-life (similar to 30 min). D2 activity is similar to 30-fold higher in Se-supplemented than in Se-deplet ed medium. An antiserum prepared against a peptide deduced from the Dio2 mR NA sequence precipitates a Se-75 protein of the predicted 31-kDa size from Se-75-labeled mesothelioma cells. Bromoadenosine 3 '5 ' cyclic monophosphat e increases D2 activity and Se-75-p31 similar to2.5-fold whereas substrate (T-4) reduces both D2 activity and Se-75-p31 similar to2-3-fold. MG132 or l actacystin (10 muM), inhibitors of the proteasome pathway by which D2 is de graded, increase both D2 activity and Se-75-p31 3-4-fold and prevent the lo ss of D2 activity during cycloheximide or substrate (T-4) exposure. Immunoc ytochemical studies with affinity-purified anti-hD2 antibody show a Se-depe ndent increase in immunofluorescence. Thus, human D2 is encoded by hDio2 an d is a member of the selenodeiodinase family accounting for its highly cata lytic efficiency in T-4 activation.