THYROTROPIN-RELEASING-HORMONE GENE-EXPRESSION IN THE ANTERIOR-PITUITARY .3. 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. T-4 or T-3 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 T-3 and T-4, but not the biologically inactive diiodothyronine, significantly stimulated TRH accumulation in AP cells. T-3 increased TRH content in a time- and dose-dependent fashion and was much more potent than T-4. Dexamethasone (Dex) also raised the content of TRH significantly. The combination of 10(-9) M T-3 and 10(-8) M Dex dramatically potentiated the effect of either treatment alone (T-3, 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. T-3, 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 T-3 and Dex treatment caused a substantial increase in the number of cells expressing pro-TRH. The results presented here demonstrate that T-3 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.