Thyroid hormone regulates expression of the thyrotropin beta-subunit gene from both transcription start sites in the mouse and rat.

Thyroid hormones suppress transcription of the gene for the beta-subunit of thyrotropin (TSH beta). Since the TSH beta gene in both the mouse and the rat contains two start sites of transcription in exon 1, we have investigated whether expression of the gene from each start site is differentially regulated by thyroid hormones in each species. RNase protection analysis was used to assay the levels of mRNA specifically transcribed from the upstream (TSS 1) and downstream (TSS 2) transcription start sites in the mouse and rat pituitary. In euthyroid and hypothyroid pituitaries there was an approximately 5-fold and 2-fold greater abundance of mRNA derived from TSS 2 than TSS 1, respectively. Hypothyroidism induced an 18- and a 9-fold increase in TSH beta gene expression from TSS 1 and TSS 2, respectively. Treatment of hypothyroid animals for 1 day with triiodothyronine (T3) reduced expression from both start sites by about 50%; after 4 days of T3 treatment, TSH beta mRNAs derived from both start sites were below detectable levels. These results were confirmed in the rat by primer extension analysis. Expression from TSS 1 in the mouse was also shown to be dependent on thyroid status using the polymerase chain reaction (PCR) technique. In contrast to previous results from primer extension studies, PCR analysis demonstrated that alternative splicing of the TSH beta RNA primary transcript can occur when transcription is initiated at the upstream start site. We conclude that, in both the mouse and the rat pituitary, expression of the TSH beta gene from both transcription start sites is regulated by thyroid hormones.