Mechanism Underlying the Aluminum-Induced Stimulation of Bone Nodule Formation by Rat Calvarial Osteoblasts

• Published in: Journal of Health Science Vol. 50; no. 1; pp. 47 - 57
• Main Authors: Kaneki, Hiroyuki, Ishibashi, Keiko, Kurokawa, Minoru, Fujieda, Masaki, Kiriu, Michiaki, Mizuochi, Shigeki, Ide, Hayao
• Format: Journal Article
• Language: English; Japanese
• Published: Tokyo The Pharmaceutical Society of Japan 01.01.2004; Pharmaceutical Society of Japan; Pharmaceutical Society of Japan, Nihon Yakugakkai
• Subjects: aluminum; bone formation; Gi protein; osteoblast; prostaglandin E2; signal transduction
• ISSN: 1344-9702; 1347-5207
• DOI: 10.1248/jhs.50.47

The signal transduction mechanism for aluminum (Al3+)-induced stimulation of bone formation and its crosstalk with the prostaglandin E2 (PGE2) signaling pathway were studied in calvarial osteoblasts from 25-week-old rats (MOB) and those from 90-week-old rats (AOB). Alkaline phosphatase activity, the rate of [3H]proline incorporation into collagenase-digestible proteins, the total area and number of mineralized bone nodules (BN) and the content of calcium in BN, which are the markers for differentiation of osteoblasts, were dose–dependently stimulated by the treatment with Al3+ at a concentration range of 10-7-10-5 M in the cultures of both MOB and AOB. The stimulatory effects of Al3+ on the differentiation markers were abolished by the pretreatment of the cells with pertussis toxin (PTX), an inhibitor of Gi protein, indicating that the effects of Al3+ are mediated through a receptor coupled with Gi protein. Al3+ increased inositol-1,4,5-triphosphate (IP3) production and intracellular concentration of Ca2+ ([Ca2+]i) in the cultures of MOB and AOB: these effects were not observed in the presence of PTX, indicating that the effects of Al3+ are mediated through the activation of phosphatidylinositol-specific phospholipase C (PI-PLC). We have previously shown that 17-phenyl-ω-trinor-PGE2, a selective agonist for an EP1 subtype of PGE2 receptor (EP1), stimulates the differentiation markers in the cultures of MOB through the activation of PI-PLC, but not in those of AOB because of the lack of EP1. The levels of the differentiation markers obtained in the presence of the EP1 agonist were increased by the addition of Al3+ in the cultures of MOB and AOB, while Al3+ increased the levels of IP3 production and [Ca2+]i in the presence of the EP1 agonist only in the cultures of AOB. These results indicate a possibility that PI-PLC molecules stimulated by the signal through Gi protein and those stimulated by the signal through EP1 belong to the same pool and that the Al3+ signal through Gi protein induces cell differentiation via a pathway(s) independent of PI-PLC in addition to that (those) dependent on the PI-PLC. We have also shown that 11-deoxy-PGE1, a selective agonist for an EP2/EP4 subtype of PGE2 receptor (EP2/EP4), inhibits cell differentiation in the cultures of both MOB and AOB. Al3+ had no effect on the basal levels of cAMP production, but the levels induced by the EP2/EP4 agonist were dose–dependently reduced by the treatment with Al3+ at a concentration range of 10-7-10-5 M. The inhibitory effect of Al3+ on adenylyl cyclase was abolished by the pretreatment with PTX. These results indicate that Al3+ suppresses adenylyl cyclase activity induced by the EP2/EP4-mediated signal through the Gi protein-coupled receptor.