Studies of the hormonal regulation of type 2 5'-iodothyronine deiodinase messenger ribonucleic acid in pituitary tumor cells using semiquantitative reverse transcription-polymerase chain reaction.

We developed a sensitive competitive RT-PCR technique for quantitating the ratio of D2 to cyclophilin messenger RNA (mRNA) and used this to study type 2 deiodinase (D2) mRNA regulation. Hyperthyroidism in rats causes a 2- to 3-fold reduction in anterior pituitary and medial basal hypothalamus (MBH). Thyroid hormone (T3) withdrawal increased the D2/cyclophilin ratio 2- to 3-fold over 48 h in both GC and GH4C1 cells. T3 additional reduced D2 gene transcription by 50% over 2 h and about 30% over the next 2 h. D2 mRNA half-life is 2 h and is not affected by T3, indicating that its effect is due to suppression of D2 gene transcription. The T3 effect did not require new protein synthesis. Longer treatment with T3 led to a maximum decrease of 70% in D2 mRNA, indicating that there is also a T3-independent transcriptional component of the D2 gene. 3,3',5'-Triiodothyronine (reverse T3) caused a slight increase D2 mRNA over 24 h but an 80-90% decrease in D2 activity, indicating that it acts posttranscriptionally. Dexamethasone, 8 Br-cAMP, and TRH also caused modest increases in D2 mRNA in pituitary tumor cells. We conclude that D2 gene transcription has both T3-dependent and T3-independent components. Thus, posttranscriptional effects of D2 substrates such as T4 will be required for complete feedback inhibition of D2 activity. The short half-life of D2 mRNA and D2 protein explains the rapid response of D2 activity to thyroid hormone administration.