Direct and indirect actions of fibroblast growth factor 2 on osteoclastic bone resorption in cultures

Fibroblast growth factor 2 (FGF-2 or basic FGF) is known to show variable a ctions on bone formation and bone resorption, This study was undertaken to elucidate the mechanisms whereby FGF-2 affects bone metabolism, especially bone resorption, using three different culture systems. FGF-2 at 10(-9) M a nd higher concentrations induced osteoclastic cell formation in the cocultu re system of mouse osteoblastic cells and bone marrow cells, and this induc tion was abrogated by nonsteroidal anti-inflammatory drugs (NSAIDs), Ca-45 release from prelabeled cultured mouse calvariae stimulated by FGF-2 (10(-8 ) M) was also inhibited by NSAIDs, and the inhibition was stronger by NSAID s, which are more selective for inhibition of cyclooxygenase 2 (COX-2) than COX-1, suggesting the mediation of COX-2 induction. COX-2 was highly expre ssed and its messenger RNA (mRNA) level was stimulated by FGF-2 in osteobla stic cells whereas it was undetectable or not stimulated by FGF-2 in cells of osteoclast lineage. To further investigate the direct actions of FGF-2 o n osteoclasts, resorbed pit formation was compared between cultures of puri fied osteoclasts and unfractionated bone cells from rabbit Long bones. FGF- 2 (greater than or equal to 10(-12) M) stimulated resorbed pit formation by purified osteoclasts with a maximum effect of 2.0-fold at 10(-11) M, and n o further stimulation was observed at higher concentrations. However, FGF-2 at 10(-9) M - 10(-8) M stimulated resorbed pit formation by unfractionated bone cells up to 9.7-fold, NS-398, a specific COX-2 inhibitor, did not aff ect the FGF-2 stimulation on purified osteoclasts but inhibited that on unf ractionated bone cells. We conclude that FGF-2 at low concentrations (great er than or equal to 10(-12) M) acts directly on mature osteoclasts to resor b bone moderately, whereas at high concentrations (greater than or equal to 10(-9) M) it acts on osteoblastic cells to induce COX-2 and stimulates hon e resorption potently.