The Paired-Domain Transcription Factor Pax8 Binds to the Upstream Enhancer of the Rat Sodium/Iodide Symporter Gene and Participates in Both Thyroid-Specific and Cyclic-AMP-Dependent Transcription

Abstract

The gene encoding the Na/I symporter (NIS) is expressed at high levels only in thyroid follicular cells, where its expression is regulated by the thyroid-stimulating hormone via the second messenger, cyclic AMP (cAMP). In this study, we demonstrate the presence of an enhancer that is located between nucleotides ؊2264 and ؊2495 in the 5-flanking region of the NIS gene and that recapitulates the most relevant aspects of NIS regulation. When fused to either its own or a heterologous promoter, the NIS upstream enhancer, which we call NUE, stimulates transcription in a thyroid-specific and cAMP-dependent manner. The activity of NUE depends on the four most relevant sites, identified by mutational analysis. The thyroid-specific transcription factor Pax8 binds at two of these sites. Mutations that interfere with Pax8 binding also decrease transcriptional activity of the NUE. Furthermore, expression of Pax8 in nonthyroid cells results in transcriptional activation of NUE, strongly suggesting that the paired-domain protein Pax8 plays an important role in NUE activity. The NUE responds to cAMP in both protein kinase A-dependent and -independent manners, indicating that this enhancer could represent a novel type of cAMP responsive element. Such a cAMP response requires Pax8 but also depends on the integrity of a cAMP responsive element (CRE)-like sequence, thus suggesting a functional interaction between Pax8 and factors binding at the CRE-like site.Cell type-specific gene transcription is often dependent on a set of transcription factors whose combination is unique to that cell type. Three transcription factors, TTF-1, TTF-2, and Pax8, have been implicated in such a control in the case of thyroidspecific transcription of the thyroglobulin and thyroperoxidase genes (13). TTF-1 is an homeodomain (HD)-containing protein, present in the developing thyroid, lung, and diencephalon (26). TTF-2, a forkhead protein, has been detected in the endoderm of the developing foregut, including the thyroid anlage, and in the anterior pituitary (41), while the paireddomain (PD) factor Pax8 is present in both the thyroid and kidney (35). The unique combination of these factors in the thyroid follicular cells strongly suggests that their interaction plays an important role in inducing a specific pattern of gene expression in these cells. Cyclic AMP (cAMP), whose intracellular level is elevated by the thyroid-stimulating hormone (TSH), is an important modulator of gene expression in thyroid cells (1,2,14,18,20,22,34,37). Nevertheless, direct roles for the thyroid-restricted factors TTF-1, TTF-2, and Pax8 in mediating the cAMP effects in thyroid cells have not yet been demonstrated. Interestingly, well-known mediators of transcriptional regulation by cAMP, such as those acting through the cAMP responsive element (CRE) sequence (5) have been proposed to be involved only in the regulation of TSH receptor gene expression (22), but no conclusive evidence on their roles in the control exerted by TSH cAMP or on other thyroidspecific genes has been pr...

Ras signaling through PI3K confers hormone-independent proliferation that is compatible with differentiation

“…Di erential e ects on Tg and NIS expression were also observed following treatment with low doses of iodide (Uyttersprot et al, 1997), suggesting that the expression of these proteins is regulated through distinct mechanisms. Interestingly, a recent report indicates that Pax-8, one of three thyroid-speci®c transcription factors , plays a prominent role in the regulation of NIS expression (Ohno et al, 1999). Therefore, it is conceivable that RasC40-dependent signals impinge selectively upon Pax-8 expression and/or activity.…”

Evidence for Transcriptional and Posttranscriptional Alterations of the Sodium/Iodide Symporter Expression in Hypofunctioning Benign and Malignant Thyroid Tumors

“…Thus, the impairment of hNIS gene expression occurring in hypofunctioning thyroid tumors could simply result from a switching off of the pathway controlling the expression of hNIS gene. In vitro studies on thyroid cell models and in vivo studies in rodents indicate that the main regulator of NIS gene expression is TSH and that the hormone primarily acts at the level of transcription, [23][24][25][26][27] but also exerts posttranscriptional regulatory actions. 21 Because patients with a cold nodule, be it an adenoma or a carcinoma, are generally euthyroid with a plasma TSH concentration within the normal range, and as TSH receptor expression is maintained in these tumors, 28 it is reasonable to think that the impairment of hNIS expression occurring in hypofunctioning thyroid tumors could result from alterations of regulatory elements along the TSH-activated pathway.…”

RET/PTC-induced dedifferentiation of thyroid cells is mediated through Y1062 signaling through SHC-RAS-MAP kinase

“…Moreover, both of these transcription factor activate the Tg and TPO promoters in heterologous cell systems (Francis-Lang et al, 1992a;Zannini et al, 1992). In contrast to Tg and TPO, expression of NIS requires factors in addition to Pax-8 and TTF-1 (Ohno et al, 1999;Chun and Di Lauro, 2001;Schmitt et al, 2002).…”