Type 2 iodothyronine deiodinase expression is upregulated by the protein kinase A-dependent pathway and is downregulated by the protein kinase C-dependent pathway in cultured human thyroid cells

Type 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze thyroxine (T4) activation. In human thyroid, unlike rodents', both DI and D2 are expressed . We have investigated the effects of thyrotropin (TSH), dibutyryl cyclic a denosine monophosphate [(Bu)(2)cAMP] (an activator of protein kinase A [PKA ]), 12-O-tetradecanoylphorbor 13-actate (TPA) (an activator of protein kina se C [PKC]), T4, and triiodothyronine (T-3) on the D2 mRNA levels and activ ity in cultured human thyroid cells. D2 mRNA levels were increased by TSH a nd (Bu)(2)cAMP, and the increment was faster and greater than that of D1 mR NA levels. The increment of the maximum velocity (V-max) value for D2 by (B u)(2)cAMP stimulation was similar to that of D2 mRNA levels, suggesting tha t (BU)(2)cAMP enhances D2 activity mainly at the pretranslational level. Cy cloheximide, a protein synthesis inhibitor, partially inhibited the increas e of D2 mRNA levels by (Bu)(2)cAMP, suggesting that de novo protein synthes is-dependent pathways are involved. TPA suppressed the D2 mRNA levels in th e presence of (Bu)(2)cAMP. However, T3 and T4 did not significantly change the D2 mRNA levels and activity. In conclusion, D2 expression in human thyr oid cells is more rapidly and strongly upregulated by the PKA pathway than D1 expression, and is downregulated by the PKC pathway.