Thyrotropin (TSH)-releasing hormone-responsive elements in the rat TSHβ gene have distinct biological and nuclear protein-binding properties

Abstract

Two TRH-responsive elements have been identified in the rat TSHβ gene by deletion/mutation analysis of the 5′-flanking region of the gene and transfection of TSHβ-luciferase constructs into the GH3 pituitary cell line. Biological responsiveness was confirmed by inserting synthetic oligonucleotides next to the heterologous viral thymidine kinase (tk) promoter in tk luciferase (tkLUC) constructs. Both DNA regions, termed TSH A (at -274 to -258 bp) and TSH C (-402 to -385 bp), have a high level of sequence similarity to binding sites for the POU domain pituitary transcription factor Pit-1/GHF-1. In transfection assays, the TSH A region had no basal enhancer activity, but did confer 3- to 6-fold TRH- and PMA-stimulated transcriptional responses to the tk promoter. The TSH C region conferred basal enhancer activity (3-to 10-fold above control tkLUC) as well as a 2- to 3-fold TRH or PMA response. Combinations of TSH A and TSH C elements conferred both enhancer activity and a TRH- or PMA-stimulated response, but more than two copies of the regions resulted in no further stimulatory effect. Both TSHβ gene regions bound to nuclear proteins from GH3 cells, as determined by gel retardation analysis. The TSH A region DNA formed three prominent DNA-protein complexes, ranging from slowly to rapidly migrating bands and with calculated affinities of 32, 0.5, and 208 nM, respectively. The TSH C region formed two major complexes, which corresponded on the basis of mobility to the most slowly and rapidly migrating complexes formed by TSH A, but with calculated affinities of 3.1 and 33 nM. TSH C also formed a rapidly migrating minor complex unique for this gene region. The more rapidly migrating complexes appeared to be specific to nuclear proteins from GH3 cells. Treatment of cells with TRH did not significantly alter the affinity of protein binding. Mutation of TSH A and TSH C DNA by T to G substitutions abolished the ability of the DNA to confer a TRH response and severely inhibited the ability of the DNA to bind to GH3 nuclear proteins. Thus, transcriptional regulation of the rat TSHβ gene by TRH is correlated with the ability of the two TRH-sensitive elements to bind nuclear proteins. The differences noted in basal enhancer activity or the degree of TRH responsiveness may be related to some unique proteins bound to each DNA or to the differences in affinity of binding of the proteins common to both elements.