1,25-DIHYDROXYVITAMIN D-3-INDUCED UP-REGULATION OF THE THYROTROPIN-RELEASING-HORMONE RECEPTOR IN CLONAL RAT PITUITARY GH(3) CELLS

1,25-Dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) is active in primary dispersed and clonal pituitary cells where it stimulates pituitary hormone production and agonist-induced hormone release. We have studied the effect of 1,25-(OH)(2)D-3 on thyrotropin-releasing hormone (TRH) binding in clonal rat pituitary tumour (GH(3)) cells. Compared with vehicle-treated cells, 1,25-(OH)(2)D-3 (10 nmol/l) increased specific [H-3]MeTRH binding by 26% at 8 h, 38% at 16 h, 35% at 24 h and reached a maximum at 48 h (90%). In dose-response experiments, specific [H-3]MeTRH binding increased with 1,25-(OH)(2)D-3 concentration and reached a maximum at 10 nmol/l. HalEmaximal binding occurred at 0.5nmol 1,25-(OH)(2)D-3/l. The vitamin D metabolite, 25-OH D-3 increased [H-3]MeTRH binding but was 1000-fold less potent than 1,25-(OH)(2)D-3. In equilibrium binding assays, treatment with 10 nmol 1,25-(OH)(2)D-3/l for 48 h increased the maximum binding from 67.4 +/- 8.8 fmol/mg protein in vehicle-treated cells to 96.7+/-12.4 fmol/mg protein in treated cells. There was no difference in apparent K-d (1.08+/-0.10nmol/l for 1,25-(OH)(2)D-3-treated and 0.97+/-0.11 nmol/l for vehicle-treated cells). Molecular investigations revealed that 10 nmol 1,25-(OH)(2)D-3/l for 24 h caused an 8-fold increase in TRH receptor-specific mRNA. Actinomycin D (2 mu g/ml, 6 h) abrogated the 1,25(OH)(2)D-3-induced increase in [H-3]MeTRH binding. Cortisol also increased [H-3]MeTRH binding but showed no additivity synergism with 1,25-(OH)(2)D-3. TRH-stimulated prolactin release was not enhanced by 1,25-(OH)(2)D-3. conclude that the active vitamin D 1,25-(OH)(2)D-3, caused a time- and dose-dependent increase in [H-3]MeTRH binding. The effect was vitamin D metabolite-specific and resulted from an upregulation of the TRH receptor. Further studies are needed to determine the functional significance of this novel finding.