THYROID-HORMONE REGULATES THE MOUSE THYROTROPIN BETA-SUBUNIT GENE PROMOTER IN TRANSFECTED PRIMARY THYROTROPES

  • 5 September 1989
    • journal article
    • research article
    • Vol. 264  (25) , 14840-14847
Abstract
In TtT 97 cells, a thyrotropin-producing mouse pituitary tumor, throid hormone rapidly inhibits the transcription rate of both the thyrotropin .alpha.- and .beta.-subunit (TSH.beta.) genes, and this closely parallels the increase in nuclear thyroid hormone receptor occupancy. In this study, we have identified regions of the mouse TSH.beta. gene which are involved in mediating tissue-specific and thyroid hormone-regulated expression. Transient expression studies were performed using a series of chimeric plasmids in which 5''-flanking DNA was ligated to the firefly luciferase gene. Following transfection by electroporation, efficient expression of TSH.beta. 5''-flanking luciferase constructs occurred only in cells derived from TtT 97 tumors which express the endogenous TSH.beta. gene. Deletion analysis demonstrated that the region of the 5''-flanking DNA between positions -271 and -80 relative to the major transcriptional start site is important for TSH.beta. promoter activity in thyrotropes. No expression was measurable in mouse L cells, a fibroblast line, whereas a low level of expression was seen in MGH 101A cells derived from a thyrotropic tumor which no longer expresses the TSH.beta. gene. Reduced expression of TSH.beta. constructs was also found in GH3 and GH4 pituitary tumor lines. Addition of thyroid hormone effectively inhibited the level of transient TSH.beta. promoter activity in TtT 97 cells in a dose-dependent manner. The inhibitory effect was more pronounced and more accurately reflected the transcription rate data when transfected cells were derived from tumors treated with thyroid hormone for 5 days prior to transfection. Deletion of all but 46 base pairs of TSH.beta. gene 5''-flanking DNA and 3 base pairs of the first exon had no effect on thyroid hormone inhibition. This indicates that signals sufficient for transcriptional regulation of the TSH.beta. gene by thyroid hormone reside in the vicinity of the proximal promoter and may act by interfering with basal transcriptional factors.