α-Difluoromethylornithine inhibits bone resorption in vitro without decreasing β-glucuronidase release

Our previous studies suggested that the ornithine decarboxylase inhibitor α-difluoromethylornithine (DFMO) inhibits bone resorption by mechanisms that are independent of polyamine depletion. To determine whether DFMO prevents calcitriol-stimulated bone resorption by acting at a step before or after osteoclast activation, we compared the effects of DFMO on release of calcium and β-glucuronidase from cultured neonatal mouse calvaria. DFMO, at concentrations of 7.5-20 mM, inhibited release of calcium from calcitriol-stimulated calvaria but failed to inhibit the calcitriol-stimulated increase in β-glucuronidase secretion. In contrast, ornithine, putrescine, spermidine, and spermine, at concentrations with effects on resorption comparable to those of DFMO, inhibited the effects of calcitriol on both calcium and β-glucuronidase release. NaF (0.2 mM), like DFMO, inhibited calcitriol-stimulated calcium release without affecting medium β-glucuronidase activity, whereas elevated phosphate (3 mM) inhibited both activities. The results suggest that DFMO, over the concentration range studied, inhibits calcium release by marking the matrix resistant to resorption rather than by acting at a cellular locus.