Characterization of a thyroid-specific enhancer located 5.5 kilobase pairs upstream of the human thyroid peroxidase gene.

Abstract

A 6.3-kbp segment of DNA, upstream of the human thyroid peroxidase gene, and various deletions thereof were linked to a promoterless bacterial chloramphenicol acetyltransferase reporter gene. These constructs were analyzed by transfection and expression in rat FRTL-5 thyroid cells and in human hepatoma HepG2 cells to localize sequences that are important for thyroid cell-specific expression of the thyroid peroxidase gene. A thyroid-specific enhancer element, capable of activating enhancerless simian virus 40 promoter expression in FRTL-5 cells, was localized to a 230-bp region approximately 5.5 kbp upstream of the human thyroid peroxidase gene transcription start site. DNase I footprinting, using nuclear extracts prepared from FRTL-5 cells, revealed three regions within the 230-bp fragment; none of these regions were protected by nuclear extracts from HepG2 cells. Gel mobility shift assays, using double-stranded oligonucleotides corresponding to the three protected regions, further confirmed the existence of factors in FRTL-5 cells, but not HepG2 cells, able to specifically bind to the enhancer sequences. These results suggest the presence of three cis-acting DNA elements in the human thyroid peroxidase gene enhancer that interact with thyroid-specific trans-acting factors.Regulation of eucaryote genes is accomplished, in part, by the binding of trans-acting protein factors to cis-acting DNA elements. Mechanisms governing tissue-or developmental stage-specific gene regulation have been extensively studied in liver-or muscle-derived cell lines (4, 29, 34). In the case of thyroid tissue, however, little is known about the mechanisms by which thyroid-specific genes are controlled. Transcription of the thyroglobulin gene, whose product is essential to thyroid hormone synthesis (9, 43), is due in part to thyroid-specific (6) and cyclic AMP-responsive (25, 35) DNA elements. However, a thyroid-specific enhancer element has not been described.Thyroid peroxidase (TPO) is a key enzyme involved in thyroid hormone synthesis (9, 43). The cDNAs encoding human (33,36,38) and rat (10, 30) TPO and the human (1, 32) and rat (15) TPO genes have been cloned and sequenced. Recently, studies on human TPO gene promoter activity have been reported which used stable transformants harboring 1.3 kbp of the 5'-flanking sequence connected to a luciferase reporter gene (14) and transient expression of a construct in which 0.9 kbp of TPO upstream DNA was linked to chloramphenicol acetyltransferase (CAT) or growth hormone genes (1). These studies were carried out in dog thyroid cell primary cultures (1). TPO gene promoter activity has also been examined by using the rat TPO gene upstream sequence of 0.4 and 3 kbp in length, fused to the luciferase gene in transient assays (15). Again, these studies did not reveal the presence of an enhancer element responsible for tissue-specific expression in thyroid cells.In this study, we prepared a construct harboring 6.

Autoregulation of thyroid-specific gene transcription by thyroglobulin

“…TPOluciferase constructs were obtained from TPO-CAT constructs (7) by placing the upstream sequence into the luciferase reporter plasmid, pSV0AL-A⌬5, which was also used to measure transfection efficiency (18,24). Rat TTF-1(T͞EBP)-luciferase constructs were prepared by PCR amplification of various lengths of the TTF-1 gene upstream sequences by using a rat genomic clone as template (6,7).…”

“…TSH, however, simultaneously increases Pax-8 binding to the TTF-1͞Pax-8 cis element of the TG promoter (10). This explains the ability of TSH to decrease TSHR, but increase TG (or TPO) gene expression, i.e., TSHR is regulated only by TTF-1, but TG and TPO are regulated by TTF-1 and Pax-8 (3)(4)(5)(6)(7)(8)(9)(10). TTF-1 increases, whereas Pax-8 decreases, major histocompatibility complex (MHC) class I gene expression in the thyroid cell (18).…”

“…Decreased TTF-1, Pax-8, and TTF-2 would decrease maximal TG and TPO gene expression (3)(4)(5)(6)(7)(8)(12)(13)(14) and decreased TTF-1 would decrease maximal TSHR and NIS gene expression (9-11, 15, 16). Decreased TTF-1 and Pax-8 would allow the cAMP response element (CRE)-binding protein (CREB) to bind to the CRE that lies between the TTF-1 and TTF-1͞Pax-8 sites on the class I 5Ј flanking region, Ϫ127 to Ϫ80 bp (18,35).…”

“…Thyroid transcription factor (TTF)-1 is essential for maximal expression of all the genes (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). TTF-1 functions together with Pax-8, a paired domain protein that binds to a sequence overlapping some TTF-1 sites, and with TTF-2, a factor that binds to an insulin-responsive element (12)(13)(14).…”

Interleukin-10 Induces Uteroglobin-related Protein (UGRP) 1 Gene Expression in Lung Epithelial Cells through Homeodomain Transcription Factor T/EBP/NKX2.1

“…Uteroglobin-related protein 1 (UGRP1) 1 was originally identified as a downstream target gene for homeodomain transcription factor T/EBP (thyroid-specific enhancer-binding protein), also called thyroid transcription factor-1 (TTF-1) or NKX2.1 (1). T/EBP regulates the expression of thyroid-and lung-specific genes including thyroglobulin (2), thyroid peroxidase (3,4), thyrotropin receptor (5), and Na/I symporter (6) in the thyroid, and surfactant proteins A (7), B (8), and C (9), and Clara cell secretory protein (10) in the lung. T/EBP is expressed in brain, thyroid, and lung epithelium during embryogenesis and is essential for the genesis of these organs (11).…”