P2 purinergic receptors potentiate parathyroid hormone receptor- mediated increases in intracellular calcium and inositol trisphosphate in UMR-106 rat osteoblasts

Abstract

The PTH receptor has been cloned and shown to activate both adenylate cyclase and phospholipase C. Evidence exists that both signaling pathways are important for mediating the net physiological effects of this hormone on bone remodeling. We have shown previously that UMR-106 osteoblastic sarcoma cells express two calcium-signaling P2 purinergic receptors, a P2U and a unique P2T receptor. Neither receptor modulates PTH receptor-mediated activation of adenylate cyclase. We now report that stimulation of either P2 receptor will, however, potentiate the magnitude of the calcium signal observed after subsequent addition of human (h) PTH-(1-34) to fluo-3-loaded UMR-106 cells. Results from experiments with staurosporine and phorbol 12-myristate 13-acetate argue against a role for protein kinase C as a mediator of this potentiating effect of P2 receptor ligands. The P2 receptor-mediated intracellular calcium elevation itself cannot account for the potentiating mechanism, because addition of ionomycin will not replicate the effect of P2 receptor ligands on hPTH-(1-34) signaling. Addition of EGTA after exposure to P2 ligands does not prevent the potentiation of hPTH-(1-34), indicating that P2 ligands potentiate the release of intracellular calcium after PTH receptor stimulation. Inositol trisphosphate production is potentiated in response to hPTH-(1-34) after first priming [3H]inositol-labeled cells with a P2 agonist. We conclude that UMR-106 cells express PTH receptors that are capable of activating adenylate cyclase, but may be unable to activate phospholipase C until cells receive a signal as a consequence of P2 receptor activation. The nature of the signal is unclear, but appears not to be mediated by either calcium or protein kinase C.