Abstract
Phenytoin is one of the most widely used anticonvulsant drugs for epileptic patients. Osteomalacia and osteoporosis are well known side effects of long-term administration of phenytoin. It is generally accepted that phenytoin mediates degradation of serum active vitamin D metabolites in the liver, and inhibition of calcium absorption in the intestine. We predicted that phenytoin stimulated bone resorption in the bone microenvironment, and attempted to determine whether phenytoin directly induced osteoclastic bone resorption utilizing reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of osteoclast differentiation factor (ODF/RANKL) and a co-culture system of osteoblasts and bone marrow cells.
Phenytoin transiently induced the expression of ODF/RANKL mRNA in osteoclastic MC3T3-E1 cells. The mRNA level reached a peak at 6h after addition of phenytoin, and returned to the basal level after 12h, and then increased again after 24h. The effect of phenytoin on ODF/RANKL mRNA was dose-dependent. The up-regulation of ODF/RANKL mRNA by phenytoin did not requirede novoprotein synthesis and involved a transcriptional mechanism. In the co-cultures of mouse bone marrow cells and osteoblasts, phenytoin markedly induced the formation of tartrate-resistant acid phosphatase-positive multinucleated osteoclast-like cells, in the presence of 10-7M 1, 25-dihydroxyvitamin D3and 10-8M dexamethasone, whose effect was significantly reversed by a neutralizing antibody against ODF/RANKL.
These results suggest that phenytoin up-regulates osteoclastogenesis, at least in part, through the induction of ODF/RANKL by osteoblasts in the local environment.
