Stimulation of osteoblast proliferation by olpadronate and its relationship to protein phosphatases

MORELLI SUSANA; SCODELARO BILBAO, PAOLA GABRIELA; KATZ, SEBASTIÁN; ROLDÁN, EMILIO; BOLAND, RICARDO; SANTILLÁN, GRACIELA

Bangkok

Congreso; International Osteoporosis Foundation and National Osteoporosis Foundation World Congress on Osteoporosis; 2008

International Osteoporosis Foundation and the National Osteoporosis Foundation of the USA

Like other bisphosphonates, olpadronate (OPD) exerts bone antiresorptive effects through inhibition of osteoclast  activity. However, direct actions of OPD on osteoblasts may take place. Within this interpretation, in the present work, we studied the presence of an OPD target in osteoblasts. Competitive binding assays using [3H]-olpadronate [3H]OPD) in whole cells (osteoblastic-like ROS 17/2.8 cell line and rat osteoblast primary cultures) showed the presence of specific, saturable and high affinity binding for OPD (Kd=1.39+0.33 μM) in osteoblasts. [3H] OPD was displaced from its binding site by millimolar concentrations (8 mM) of the non-permeant protein phosphatase (PP) substrates nitrophenylphosphate and α-naphtylphosphate, whereas PP inhibitors (0.2–8 mM) orthovanadate, NaF or vpb(bipy) did not displace [3H]OPD. As expected, specific OPD binding was detected in the plasma membrane, although significant BP binding was also detected intracellularly. Of relevance, OPD increased DNA synthesis in ROS 17/2.8 cells with a temporal profile similar to that of protein tyrosine phosphatase (PTP) inhibitors Na3VO4 and vpb(bipy), but different from NaF, a general PP inhibitor. The stimulatory effect of OPD and PTP inhibitors on osteoblast proliferation was inhibited by the protein tyrosine kinase (PTK) inhibitors genistein and geldanamycin. These results provide new evidence on the existence of an OPD target site in osteoblasts, presumably a PTP, which may be involved in the stimulatory action of OPD on osteoblast proliferation, thereby, suggesting an anabolic effect of this bisphosphonate.