THYROTROPE EXPRESSION AND THYROID-HORMONE INHIBITION MAP TO DIFFERENT REGIONS OF THE MOUSE GLYCOPROTEIN HORMONE ALPHA-SUBUNIT GENE PROMOTER

The α-subunit gene of the glycoprotein hormones is normally expressed in pituitary thyrotropes and gonadotropes and in placental cells. Thus, this gene must contain elements that mediate expression and hormonal responses in different cell types. The localization of DNA regions important for expression and regulation of the α-subunit gene in thyrotrope cells has not previously been reported. In these studies luciferase expression constructs containing 1700 basepairs of 5’ flanking DNA derived from the mouse a-subunit gene were introduced by electroporation into freshly dispersed cells from TSH-producing mouse pituitary tumors (TtT 97). This promoter functioned with greater efficiency in thyrotropes than in nonthyrotrope pituitary GH4cells and L-cell fibroblasts. Primer extension confirmed that transcription from the a-subunit constructs initiated at the same site as the endogenous gene. Studies using 5’ truncations showed a progressive loss of α-subunit promoter activity in thyrotropes between —480 and —120, with regions upstream of —254 contributing substantially to expression in thyrotrope cells. Thyroid hormone inhibited α-subunit promoter activity in a dose-dependent fashion, although in vivo treatment of tumors with thyroid hormone before transfection was necessary to achieve maximal inhibition. Thyroid hormone inhibition of α-subunit promoter activity also occurred in GH4cells, but no effect was observed in L-cells. Studies using 5’ truncations localized a region responsible for thyroid hormone inhibition between —62 and +43, encompassing the TATA sequence and the transcriptional initiation site. When this region was compared to the thyroid hormone inhibitory regions of the α-subunit genes from other species and the mouse TSHβ-subunit gene, a 6-basepair motif, 5’ (G/A)GTG(G/A)G 3’, emerged as a possible consensus sequence for a thyroid hormone inhibitory element. © 1990 by The Endocrine Society.