Abstract
The effect of prostaglandins (PG) on free cytosolic calcium concentrations [( Ca2+]i) and cAMP levels was studied in the osteosarcoma cell line UMR-106. PGF2 alpha and PGE2, but not 6-keto-PGF1 alpha, induced an increase in [Ca2+]i which was mainly due to Ca2+ release from intracellular stores. The EC50 for PGF2 alpha was approximately 7 nM, whereas that for PGE2 was approximately 1.8 microM. Maximal doses of PGF2 alpha increased [Ca2+]i to higher levels than PGE2. Both active PGs also stimulated phosphatidylinositol turnover in UMR-106 cells. The effects of the two PGs were independent of each other and appear to involve separate receptors for each PG. PGE2 was a very potent stimulator of cAMP production and increased cAMP by approximately 80-fold with an EC50 of 0.073 microM. PGF2 alpha was a very poor stimulator of cAMP production; 25 microM PGF2 alpha increased cAMP by 5-fold. The increase in cellular cAMP levels activated a plasma membrane Ca2+ channel which resulted in a secondary, slow increase in [Ca2+]i. High concentrations of both PGs (10-50 microM) inhibited this channel independent of their effect on cAMP levels. Pretreatment of the cells with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate inhibited the PG-mediated increase in phosphatidylinositol turnover and the increase in [Ca2+]i. However, pretreatment with 12-O-tetradecanoyl-13-acetate had no effect on the PGE2-mediated increase in cAMP. The latter finding, together with the dose responses for PGE2-mediated increases in [Ca2+]i and cAMP levels, suggests the presence of two subclasses of PGE2 receptors: one coupled to adenylate cyclase and the other to phospholipase C. With respect to osteoblast function, the cAMP signaling system is antiproliferative, whereas the Ca2+ messenger system, although having no proliferative effect by itself, tempers cAMP's antiproliferative effect.
Highlights
The effect of prostaglandins (PG) on free cytosolic tivity on arachidonic acid, are produced by many cell types calcium concentrations ([Ca”’],) and cAMP levels was studied in theosteosarcoma cell line UMR-106
Specific plasma membrane PG receptors have been identified in many cell types [8,9].Binding of PGs can either increase [10,11,12] or decrease [13,14,15] cAMP levels in cells, which is correlated with their biological effects
The effect of PGs on bone cell metabolism was studied in organ culture and freshly isolated bone cells
Summary
Relationship of cAMP and Calcium MessengerSystems in Prostaglandin-stimulatedUMR- 106 Cells*. PGs are produced and secreted by osteoblasts [9,11,12] andosteosarcoma cell lines [17] and are potent stimulators of bone resorption [Ca2+li.High concentrations of both PGs (10-50 p ~ )(18-23) and inhibitors of collagen synthesis by osteoblasts inhibited this channel independent of their effect on cAMP levels. On the day of experimentation, the unincorporated [3H]myoinositol was washed away with serum-free Trowell's T8 medium with the above additives and kept in that same medium for 1h inthe presence of 10 mM LiCl. Cells were stimulated with the PGs or vehicle (100% ethanol), with or without TPA, for the designated times.
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