Acute phosphate depletion dissociates hormonal stimulated second messengers in osteoblast-like cells.
The acute effect (24 h) of either phosphate depletion or phosphate surfeit on hormonal stimulated signal transduction systems was studied in the osteoblastic cell line UMR-106. Elevation of intracellular Ca2+ ([Ca2+]in), induced by different calciotropic hormones (PTH, prostaglandin E2, endothelin) was blunted by acute phosphate depletion, whereas at high inorganic phosphate (Pi) concentrations the rise in [Ca2+]in was augmented. Basal [Ca2+]in was not altered by either Pi depletion or Pi excess. The effect of acute phosphate depletion on hormonal mediated [Ca2+]in rise was not observed in the absence of extracellular Ca2+ suggesting that under these conditions, the release of Ca2+ from intracellular stores, is not affected. Also, nonhormonal calcium entry pathways such as depolarization-activated calcium channels or protein kinase C-activated Ca2+ channels were not affected by acute phosphate depletion. cAMP accumulation in the cells, either through receptor or nonreceptor-mediated mechanisms, increased under low Pi conditions and decreased as Pi concentration in the culture media was progressively increased from 0 to 2 mM during 24 h of incubation. Changes in Pi concentration had no effect on basal cAMP generation by the cells. The facilitative effect of acute Pi depletion on agonist-induced cAMP accumulation could be demonstrated in both the presence and absence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.2 mM). PTH receptor binding assessed with [Nle8 Nle18 Tyr34] bovine PTH (1-34) NH2 was not altered by phosphate depletion. We conclude that exposure of osteoblasts to different Pi environments modulates the second messenger responses to hormones in a reciprocal fashion so that acute phosphate depletion down-regulates [Ca2+]in signals while augmenting cAMP generation and vice versa. Inasmuch as bone resorption processes can be modulated by Ca2+ and cAMP the data presented herein suggest that the altered bone resorptive response to calciotropic hormones (e.g. PTH), under surfeit or deficit of phosphate, is mediated by changes in [Ca2+]in and cAMP.
Green
J
J
Laboratory of Membrane Biology, Cedars-Sinai Medical Center, University of California, School of Medicine, Los Angeles 90048.
Kleeman
C R
CR
Schotland
S
S
Chaimovitz
C
C
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D015232:Dinoprostone / Q000494:pharmacology*; D016232:Endothelins / Q000494:pharmacology*; D010006:Osteoblasts / Q000187:drug effects / Q000378:metabolism*; D012516:Osteosarcoma; D010281:Parathyroid Hormone / Q000494:pharmacology*; D010446:Peptide Fragments / Q000494:pharmacology*; D010710:Phosphates / Q000008:administration & dosage* / Q000494:pharmacology; D015290:Second Messenger Systems / Q000187:drug effects*; D015398:Signal Transduction / Q000187:drug effects; D014407:Tumor Cells, Cultured
D016232:Endothelins; D010281:Parathyroid Hormone; D010446:Peptide Fragments; D010710:Phosphates; C025076:parathyroid hormone (1-34), bovine; D000242:Cyclic AMP; D015232:Dinoprostone; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine