1,25-dihydroxyvitamin D-3 and 24,25-dihydroxyvitamin D-3 affect parathormone (PTH) -sensitive adenylate cyclase activity and alkaline phosphatase secretion of osteoblastic cells through different mechanisms of action.
In UMR 106 rat osteosarcoma cells, parathormone (1-34hPTH) and calcitonin (sCT) stimulated adenylate cyclase (AC) activity 5.5-and 2.8-fold, respectively. AC in osteoblasts (OB) from collagenase-treated calvaria of 3-day-old rats responded similarly to 1-34hPTH. In contrast, fibroblasts (mouse fibroblastomas) displayed a marginal 1-34hPTH sensitive AC. Osteoclasts (OC) of collagenase-treated rat calvariae, rat monocytes and mouse macrophages did not demonstrate 1-34hPTH inducable AC activity. Physiological concentrations of 24,25-dihydroxyvitamin D-3 attenuated PTH-sensitive AC in OB and UMR 106 cells within 20 min, while 1,25-dihydroxyvitamin D-3 showed no such immediate effect. In contrast, the AC response to Gpp(NH)p was unaffected by 24,25-(OH)2D3, indicating that 24,25-(OH)2D3 interrupts the coupling of the PTH receptor to the GTP binding protein Gs. OB and UMR 106 cells were also subjected to long-term (48 h) incubation with vitamin D-3 metabolites, 1-34hPTH or 20% serum from patients with secondary hyperparathyroidism (sHBT-serum), respectively. PTH-sensitive AC was markedly attenuated by pre-exposure to both 1-34hPTH and 1,25-(OH)2D3, while minimally affected by corresponding 24,25-(OH)2D3 and 20% sHPT-serum treatment. The secretion of alkaline phosphatase (Alphos) from the two cell types was strongly increased by 1-34hPTH, the effect being abolished by the presence of 24,25-(OH)2D3. Iliac crest biopsies of normal individuals exhibited a clear negative correlation between PTH-sensitive AC and corresponding serum 24,25-(OH)2D3 levels. Basal AC activity was, however, negatively correlated to serum 1,25-(OH)2D3 concentrations. In summary, the results show that 24,25-(OH)2D3 reduces PTH-stimulated AC activity in and Alphos secretion from osteoblastic bone cells by rapidly and directly interfering with the plasma membrane. These data reinforce the probable in vivo significance of 24,25-(OH)2D3. Moreover, the negative correlation between basal AC activity and serum 1,25-(OH)2D3 levels indicates a possible role for 1,25-(OH)2D3 in regulating bone cell synthesis of AC components in vivo.
Klem
K H
KH
Institute for Surgical Research, National Hospital, Oslo, Norway.
Jablonski
G
G
Saether
O
O
Jarosz
G
G
Gautvik
K M
KM
Gordeladze
J O
JO
D015650:24,25-Dihydroxyvitamin D 3 / Q000494:pharmacology*; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000469:Alkaline Phosphatase / Q000378:metabolism*; D000818:Animals; D002116:Calcitonin / Q000494:pharmacology; D002117:Calcitriol / Q000494:pharmacology*; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000096:biosynthesis; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D066298:In Vitro Techniques; D010006:Osteoblasts / Q000201:enzymology*; D010281:Parathyroid Hormone / Q000494:pharmacology*; D051381:Rats; D014407:Tumor Cells, Cultured
D010281:Parathyroid Hormone; D005576:Colforsin; D006165:Guanylyl Imidodiphosphate; D015650:24,25-Dihydroxyvitamin D 3; D002116:Calcitonin; D000242:Cyclic AMP; D000469:Alkaline Phosphatase; D000262:Adenylyl Cyclases; D002117:Calcitriol