Regulation of the Transfected Human Glycoprotein Hormone α-Subunit Gene by Dexamethasone and Thyroid Hormone

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Abstract
A chimeric plasmid, (-1,500)h.alpha.CAT, containing approximately 1,500 bp of 5''-flanking DNA of the human glycoprotein hormone .alpha.-subunit gene directing the expression of the bacterial chloramphenicol acetyl transferase (CAT) gene, was transfected transiently into rat pituitary-derived GH3 cells. (-1,500)h.alpha.CAT expression was stimulated 5- to 20-fold by dexamethasone and 3- to 5-fold by 8-bromo-cAMP (8-Br-cAMP), and was inhibited by 50% by L-triiodothyronine (T3). Thus, suppression by T3 in this system was similar to that seen in pituitary thyrotropes. Induction of (1,500)h.alpha.CAT expression by dexamethasone was antagonized by T3 but was unaffected by 8-Br-cAMP. However, T3 augmented the stimulation of (-1,500)h.alpha.CAT activity by 8-Br-cAMP. Deletants containing less than 346 bp of 5''flanking .alpha.DNA showed a stepwise decrease in induction by dexamethasone, suggesting that multiple sequence elements located in this region are required for full induction of h.alpha.CAT activity. Deletion analysis also indicated that a thyroid hormone response element is located between 207 and 172 bp of the .alpha. gene transcriptional start site. Our finding of induction of .alpha. expression by dexamethasone in pituitary cells contrast with the inhibition of .alpha. gene activity by glucocorticoids which has previously been shown in placental cells. Therefore, these data indicate that cell-type-specific factors play an important role in the modulation of .alpha. gene transcription.