REGULATION OF THYROID-STIMULATING HORMONE BETA-SUBUNIT AND GROWTH-HORMONE MESSENGER-RIBONUCLEIC-ACID LEVELS IN THE RAT - EFFECT OF VITAMIN-A STATUS

T-3 inhibits transcription of the rat TSH beta gene, and two T-3 respo nse elements have been identified that bind T-3 receptors and that sha re sequence homology with the consensus sequence that is also recogniz ed by retinoic acid receptors (RARs). We, therefore, asked whether RA was a regulator of TSH beta gene expression in vivo. Using RNase prote ction analysis, we found that vitamin A deficiency led to a a-fold inc rease in rat pituitary TSH beta messenger RNA (mRNA) levels, which ret urned to normal 18 h after treatment with RA(20 mu g/rat). Vitamin A d eficiency had no effect on TSH beta mRNA levels in hypothyroid rats. C oadministration of RA and T-3 (10 mu g/100g body wt) to either vitamin A-deficient or vitamin A-deficient, hypothyroid animals caused decrea ses in TSH beta mRNA. content that were indistinguishable from those s een with T-3 alone. Surprisingly, vitamin A deficiency had no signific ant effect on GH mRNA levels in euthyroid or hypothyroid rats. Further more, treatment of vitamin A-deficient, hypothyroid animals with RA fo r either 18 or 72 h had no effect on GH mRNA levels, whereas T-3 cause d 11-fold and 18-fold increases in GH mRNA, respectively, at these tim es. We also used transient transfection to test for direct, retinoid r eceptor-mediated regulation of TSH beta gene transcription by RA. A pl asmid TSH beta luciferase, containing 0.8 kilobases of rat TSH beta ge ne 5'-flanking sequences, exon 1, and 150 base pairs of intron 1, was transfected into CV-1 cells. Cotransfection with RAR alpha and retinoi d X receptor-beta induced TSH beta expression by 3.5-fold, and treatme nt with RA suppressed this induction by 46%. These results show that v itamin A levels play a significant role in regulating the expression o f the TSH beta gene, but not the GH gene, in vivo and suggest that RA may suppress TSH beta gene transcription directly by an RAR-retinoid X receptor heterodimer-mediated mechanism.