Thyroid transcription factor 1 phosphorylation is not required for proteinkinase A-dependent transcription of the thyroglobulin promoter

Thyroid transcription factor 1 (TTF1) is a nuclear homeodomain protein that binds to and activates the promoters of several thyroid-specific genes, in cluding that of the thyroglobulin gene (pTg). These genes are also positive ly regulated by thyroid-stimulating hormone/cyclic AMP (cAMP)/protein kinas e A (PKA) signaling. We asked whether PKA directly activates TTF1. We show that cAMP/PKA activates pTg and a synthetic target promoter carrying TTF1 b inding site repeats in several cell types. Activation depends on TTF1. Phos phopeptide mapping indicates that TTF1 is constitutively phosphorylated at multiple sites, and that cAMP stimulated phosphorylation of one site, serin e 337, in vivo. However, alanine substitution at this residue or at ail sit es of phosphorylation did not reduce PKA activation of pTg. Thus, PKA stimu lates TTF1 transcriptional activity in an indirect manner, perhaps by recru iting to or removing from the target promoter another regulatory factor(s).