About the mechanism of down regulation of Na + /I – symporter expression in thyroid cancer

Topic: Regulation of Expression of Na + /I Symporter

Title: PTTG and PBF repress the human sodium iodide symporter.

Authors: Boelaert K, Smith VE, Stratford AL, Kogai T, Tannahill LA, Watkinson JC, Eggo MC, Franklyn JA, & McCabe CJ.

Reference: Oncogene (advance online publication; 5 February 2007); doi: 10.1038/sj.onc.1210221

Summary

Background

The ability of the thyroid gland to accumulate iodide provides the basis for radioiodine ablation of differentiated thyroid cancers and their metastases. Most thyroid tumours exhibit reduced iodide uptake, although the mechanisms accounting for this remain poorly understood. It was previously reported that -pituitary tumour transforming gene- (PTTG) is overexpressed in thyroid tumours and downregulates the expression of the sodium iodide symporter (NIS) in FRTL-5 cells.

Purpose

The authors investigated the mechanisms by which PTTG and its binding factor -PBF- could repress NIS expression.

Material and methods

A small collection of human thyroid tumors and human or rat thyroid cells in vitro: human thyrocytes in primary culture and FRTL-5 cells. The main approach was cell transfection either with expression vectors containing PTTG or PBF cDNAs or with plasmid constructs with NIS promoter region of different size and luciferase as the reporter gene.

Results

Using a series of 24 tumor samples, the authors showed that the expression of NIS was largely decreased (at both mRNA & protein levels, by western blot and immuno-histochemistry) in follicular and papillary thyroid carcinomas. Transient expression of PTTG and PBF alone or in combination in human thyrocytes decreased NIS transcript content by 80-90% and lowered iodide uptake by more than 50% within 48 hours. Studies of the NIS promoter in rat FRTL-5 cells revealed that PTTG and PBF were capable of inhibiting the promoter activity via the human upstream enhancer element (hNUE). Within this -1 Kb element, a complex PAX8-USF1 (Upstream stimulating factor 1) response element was proved to be critical both for basal promoter activity and for PTTG and PBF repression of NIS. Repression by PTTG was contingent upon the USF1 site but not the PAX8 site. In human primary thyroid cells, PTTG and PBF similarly repressed the NIS promoter.

Conclusions

These data suggest that the over-expression of PTTG and PBF, which occurs in differentiated thyroid cancer, a) could have implications on the level of expression and thus the activity of NIS, and b) hence might have a significant impact on the efficiency of radioiodine treatment.

Commentary

The understanding of the mechanisms through which NIS expression is repressed in thyroid tumors is of importance because it could open new possibilities of improvement of 131 Iodide delivery to tumors with low or no radioiodide uptake activity. It is now well documented that NIS expression is primarily regulated at the level of transcription. The mechanisms of regulation operating on the transcription of the NIS gene are likely more complex than those governing transcription of other thyroid-specific genes (Tg, TPO, -). Factors interacting directly or indirectly with the most important regulatory region of the NIS gene named NUE (for NIS Upstream Enhancer) are only partially known. Pax-8 is probably the crucial regulatory factor that binds to NUE but its expression and function does not seem to be altered in thyroid carcinomas. The present article examines the involvement of two candidate proteins known to down regulate NIS expression: PTTG and PBF.

PTTG, initially found expressed in pituitary tumors, was reported to be over-expressed in thyroid tumors by Heaney et al. in 2001. It is a 202-amino acid protein, partially localized in the nucleus probably because of its interaction with another small protein, PBF (PTTG-binding factor) possessing a nuclear localization signal. Both PTTG and PBF have been shown to have cell transforming activities. When over-expressed in mouse 3T3 fibroblasts, they induce cell transformation and injection of transfected 3T3 cells into athymic nude mice resulted in tumor formation.

The present study indicates that PTTG & PBF could repress NIS gene transcription by interfering with the binding of two transcriptional regulators, Pax-8 and USF1, with the NIS -promoter- region. This mechanism must be ascertained by protein-DNA binding experiments. Finally on a personal note, it is regrettable that the cited literature is biased in this article. For instance, articles from the group of Roberto Di Lauro, reporting the definition of NUE, were not mentioned.

( Summary and commentary prepared by Bernard Rousset )

Present summary & commentary are related to the Chapter N- 2 of TDM

Full paper obtainable at: http://www.nature.com/onc/journal/vaop/ncurrent/abs/1210221a.html;jsessionid=3834C1A85D9973F815B5F191ED8126B1