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

Abstract

Type 1 and 2 iodothyronine deiodinases (D1 and D2) catalyze thyroxine (T4) activation. In human thyroid, unlike rodents', both D1 and D2 are expressed. We have investigated the effects of thyrotropin (TSH), dibutyryl cyclic adenosine monophosphate [(Bu)₂cAMP] (an activator of protein kinase A [PKA]), 12-O-tetradecanoylphorbor 13-actate (TPA) (an activator of protein kinase C [PKC]), T₄, and triiodothyronine (T₃) on the D2 mRNA levels and activity in cultured human thyroid cells. D2 mRNA levels were increased by TSH and (Bu)₂cAMP, and the increment was faster and greater than that of D1 mRNA levels. The increment of the maximum velocity (V_max) value for D2 by (Bu)₂cAMP stimulation was similar to that of D2 mRNA levels, suggesting that (Bu)₂cAMP enhances D2 activity mainly at the pretranslational level. Cycloheximide, a protein synthesis inhibitor, partially inhibited the increase of D2 mRNA levels by (Bu)₂cAMP, suggesting that de novo protein synthesis-dependent pathways are involved. TPA suppressed the D2 mRNA levels in the presence of (Bu)₂cAMP. However, T₃ and T₄ did not significantly change the D2 mRNA levels and activity. In conclusion, D2 expression in human thyroid cells is more rapidly and strongly upregulated by the PKA pathway than D1 expression, and is downregulated by the PKC pathway.