Identification of novel genetic elements controlling transcriptional regulation of the human Na(formula)/I(formula) symporter (NIS) gene
Author
Alotaibi, Hani
Advisor
Tazebay, Uygar H.
Date
2006Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
The function of sodium iodide symporter (NIS) in mammary gland epithelial cells is
essential for the accumulation of iodide in mother’s milk, which is the first source of
iodide for the synthesis of thyroid hormones in the newborn. In addition to the
lactating mammary gland, NIS expression has been also detected in breast tumors.
Several hormones and ligands have been implicated in the functional expression of
NIS in the mammary gland and breast cancer cell line models but the molecular
determinants governing this expression are not yet identified. In this study we aimed
to identify cis- and trans-acting elements regulating NIS expression in the breast
cancer cell line MCF-7 in response to all-trans-retinoic acid (tRA), and to assess the
possible role of 17-β-estradiol (E2) in regulating the expression of NIS. Using
comparative bioinformatics, we have identified several regions that were conserved
in human, mouse and rat in the sequences flanking and including the NIS gene. By
using luciferase reporter assays, we have established that conserved clusters 3 and 4
respond to tRA in MCF-7. We have also shown that putative retinoic acid response
elements controlling tRA-induced NIS expression in MCF-7 are located in the first
intron of this gene. This tRA-responsive NIS expression was also correlated with the
estrogen receptor status of mammary gland cell lines and we investigated roles of
ERα in the regulation of NIS expression. We showed that the suppression of
endogenous ERα by RNA interference resulted in down-regulation of both basal and
tRA-induced NIS expression in MCF-7, furthermore, we have also shown that (E2)
is capable of up-regulating NIS expression in MCF-7. In the ERα negative cell line
MDA-MB-231, re-introduction of ERα resulted in NIS expression in a ligand
independent manner. The role of ERα in the regulation of NIS expression was
supported by the identification of an estrogen response element (ERE) in the
promoter of NIS, this ERE was conserved in human, mouse and rat. We have also
showed that this ERE could respond to E2 stimulation, and that ERα occupies the
NIS promoter by binding to this novel element in vivo. These results indicate that E2
and ERα contribute to the regulation of NIS in the breast cancer cell line MCF-7.