Regulation of thyroid-stimulating hormone β-subunit and growth hormone messenger ribonucleic acid levels in the rat: Effect of vitamin A status

T₃ inhibits transcription of the rat TSHβ gene, and two T₃ response elements have been identified that bind T₃ receptors and that share sequence homology with the consensus sequence that is also recognized by retinoic acid receptors (RARs). We, therefore, asked whether RA was a regulator of TSHβ gene expression in vivo. Using RNase protection analysis, we found that vitamin A deficiency led to a 2-fold increase in rat pituitary TSHβ messenger RNA (mRNA) levels, which returned to normal 18 h after treatment with RA (20 μg/rat). Vitamin A deficiency had no effect on TSHβ mRNA levels in hypothyroid rats. Coadministration of RA and T₃ (10 μg/100g body wt) to either vitamin A-deficient or vitamin A-deficient, hypothyroid animals caused decreases in TSHβ mRNA content that were indistinguishable from those seen with T₃ alone. Surprisingly, vitamin A deficiency had no significant effect on GH mRNA levels in euthyroid or hypothyroid rats. Furthermore, treatment of vitamin A-deficient, hypothyroid animals with RA for either 18 or 72 h had no effect on GH mRNA levels, whereas T₃ caused 11-fold and 18-fold increases in GH mRNA, respectively, at these times. We also used transient transfection to test for direct, retinoid receptor-mediated regulation of TSHβ gene transcription by RA. A plasmid TSHβ luciferase, containing 0.8 kilobases of rat TSHβ gene 5'-flanking sequences, exon 1, and 150 base pairs of intron 1, was transfected into CV-1 cells. Cotransfection with RARα and retinoid X receptor-β induced TSHβ expression by 3.5-fold, and treatment with RA suppressed this induction by 46%. These results show that vitamin A levels play a significant role in regulating the expression of the TSHβ gene, but not the GH gene, in vivo and suggest that RA may suppress TSHβ gene transcription directly by an RAR-retinoid X receptor heterodimer-mediated mechanism.