Pax 8 expression in primary cultured dog thyrocyte is increased by cyclic AMP

Pax 8 proteins are paired domain-containing transcription factors expressed in thyroid, kidney, ovary, placenta and developing brain. Thyroglobulin (Tg) and thyroperoxidase (TPO) genes, which are specifically expressed in thyroid follicular cells, both harbor a Pax 8 binding site in their proximal promoter region. The transcription of these genes is, as is the expression of most of the other differentiated functions of the thyrocyte, positively regulated by thyrotropin (TSH) via a cyclic-AMP (cAMP)-dependent mechanism. However, no typical cAMP-responsive element has been detected in the promoter region of Tg and TPO genes. We therefore investigated whether Pax 8 activity itself could be regulated by cAMP, which would support a role for these factors in the cAMP-dependent expression of differentiation in thyroid cells. In this paper we show that the expression of Pax 8 mRNA and proteins are increased by treatment of the thyrocyte with forskolin. This suggests that Pax 8 could indeed participate in the mediation of the transcriptional activation of thyroid specific genes by cAMP. We also show that Pax 8 are nuclear phosphoproteins, although neither their phosphorylation, nor their nuclear translocation seem to be highly regulated by cAMP. During the course of this study, a new splicing variant of dog Pax 8, termed Pax 8g, has been isolated.