Acute Phosphate Depletion Dissociates Hormonal Stimulated Second Messengers in Osteoblast-Like Cells*

Abstract

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.

Fluoride increases net45Ca uptake by SaOS-2 cells: The effect is phosphate dependent

“…Second, both classes of effects also had similar dependencies for phosphate--in both cases, an increase in the phosphate concentration in the medium was associated with a shifting of the fluoride dose-response curve to a range of lower levels. (Previous studies [15,39,40] have also reported similar P~ dependencies for 45Ca uptake and 3[H]-thymidine incorporation, and one recent study indicates that hormonemediated second messenger response (including effects on intracellular Ca) can be affected acutely by changes in the Pi concentration [41].) Finally, coincubation with verapamil prevented the actions of fluoride to increase net 45Ca uptake and 3[Hl-thymidine incorporation.…”

Interleukin-6 attenuates agonist-mediated calcium mobilization in murine osteoblastic cells.

“…However, in the absence of more direct assays of src activity, the possibility ofother intracellular TKs mediating 160-140-0 = 120-11 10 9 -LOG [PTHJ osteoblasts. Others and we have shown that the pleiotropic functions of these hormones on bone metabolism can be explained on the basis of antagonistic effects between the Ca2+ and cAMP signaling pathways (41)(42)(43)(44)56). With respect to cell growth, the cAMP messenger system is antiproliferative, while…”

“…It is now well established that the osteoblast plays a key role in both bone formation and bone resorption processes stimulated by osteotropic agents (39,40). Moreover, the Ca2' and cAMP messenger systems generated inside the osteoblastic cytosol mediate many of the effects of calciotropic hormones (41)(42)(43)(44). Given the in vivo observation of cooperativity between IL-6 and calciotropic hormones on bone remodeling, it is conceivable that the cytokine impinges on transduction pathways generated by hormones.…”

Interleukin-1B and 6-Induced Calcium Channel Current Modulation in MC3T3-E1 Osteoblast Cell Line

“…In osteoblastic cells, several studies demonstrated that hormones modulated VSCCs, included in parathyroid hormone (PTH) (Green et al, 1991;Li et al, 1997;Yamaguchi et al, 1987), bradykinin (Tokuda et al, 1994), 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) (Bergh et al, 2006;Uchida et al, 2012;Zanello et al, 2006) and phalloidin, a stabilizer of actin filaments (Li et al, 2011). To our knowledge, the data presented here demonstrate for the first time that IL-1β and -6 modulates VSCCs in osteoblasts.…”