1,25-Dihydroxyvitamin D3-induced upregulation of the thyrotropin-releasing hormone receptor in clonal rat pituitary GH3 cells.
1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) 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)2D3 on thyrotropin-releasing hormone (TRH) binding in clonal rat pituitary tumour (GH3) cells. Compared with vehicle-treated cells, 1,25-(OH)2D3 (10 nmol/l) increased specific [3H]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 [3H]MeTRH binding increased with 1,25-(OH)2D3 concentration and reached a maximum at 10 nmol/l. Half-maximal binding occurred at 0.5 nmol 1,25-(OH)2D3/l. The vitamin D metabolite, 25-OH D3, increased [3H]MeTRH binding but was 1000-fold less potent than 1,25-(OH)2D3. In equilibrium binding assays, treatment with 10 nmol 1,25-(OH)2D3/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 Kd (1.08 +/- 0.10 nmol/l for 1,25-(OH)2D3-treated and 0.97 +/- 0.11 nmol/l for vehicle-treated cells). Molecular investigations revealed that 10 nmol 1,25-(OH)2D3/l for 24 h caused an 8-fold increase in TRH receptor-specific mRNA. Actinomycin D (2 micrograms/ml, 6 h) abrogated the 1,25-(OH)2D3-induced increase in [3H]MeTRH binding. Cortisol also increased [3H]MeTRH binding but showed no additivity or synergism with 1,25-(OH)2D3. TRH-stimulated prolactin release was not enhanced by 1,25-(OH)2D3. We conclude that the active vitamin D metabolite, 1,25-(OH)2D3, caused a time- and dose-dependent increase in [3H]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.
Atley
L M
LM
University of Melbourne, Department of Medicine, Royal Melbourne Hospital, Parkville, Victoria, Australia.
Lefroy
N
N
Wark
J D
JD
D000818:Animals; D015152:Blotting, Northern; D002117:Calcitriol / Q000494:pharmacology*; D002460:Cell Line; D002478:Cells, Cultured; D002999:Clone Cells; D004305:Dose-Response Relationship, Drug; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism; D010911:Pituitary Neoplasms / Q000378:metabolism* / Q000473:pathology; D011388:Prolactin / Q000032:analysis / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D012333:RNA, Messenger / Q000032:analysis; D011863:Radioimmunoassay; D051381:Rats; D018025:Receptors, Thyrotropin-Releasing Hormone / Q000235:genetics / Q000378:metabolism*; D013997:Time Factors; D015854:Up-Regulation
D012333:RNA, Messenger; D018025:Receptors, Thyrotropin-Releasing Hormone; D011388:Prolactin; D002117:Calcitriol