Thyrotropin-releasing hormone gene expression in the anterior pituitary. III. Stimulation by thyroid hormone: Potentiation by glucocorticoids

The present study was designed to investigate the effect of thyroid hormone on TRH gene expression in cultured anterior pituitary (AP) cells. AP cells derived from 15-day-old male rats were cultured for up to 14 days in Dulbecco's Modified Eagle's Medium-L-15 medium supplemented with either fetal calf serum (FCS) or FCS devoid of thyroid hormones. T4 or T3 were added at various concentrations to the medium for a duration of 2-14 days. TRH and GH were measured by RIA, and pro-TRH mRNA levels were determined by semiquantitative in situ hybridization. Addition of both T3 and T4, but not the biologically inactive diiodothyronine, significantly stimulated TRH accumulation in AP cells. T3 increased TRH content in a time- and dose- dependent fashion and was much more potent than T4. Dexamethasone (Dex) also raised the content of TRH significantly. The combination of 10-9 M T3 and 10-8 M Dex dramatically potentiated the effect of either treatment alone (T3, 8.9-fold rise; Dex, 37.2-fold rise) and increased TRH accumulation 251.2-fold (all P < 0.01). Levels of pro-TRH mRNA mirrored TRH content data. T3, Dex, or the combination of both raised pro-TRH mRNA levels 1.9-, 2.7 (both P < 0.05)-, and 11.1 (P < 0.01)-fold, respectively. The visualization of pro-TRH mRNA by in situ hybridization revealed that the combination of T3 and Dex treatment caused a substantial increase in the number of cells expressing pro-TRH. The results presented here demonstrate that T3 increases pro-TRH gene expression in cultured AP cells and that glucocorticoids markedly potentiate this effect. As pro-TRH is expressed in a subpopulation of somatotrophs, our data suggest that the TRH gene in this location may be coordinately regulated with the GH gene.

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