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Alpha-difluoromethylornithine inhibits bone resorption in vitro without decreasing beta-glucuronidase release
P H Stern1, R C Lucas, J Seidenfeld
1Department of Pharmacology, Northwestern University, Chicago, Illinois 60611.
Abstract:
Our previous studies suggested that the ornithine decarboxylase inhibitor alpha-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 beta-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 beta-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 beta-glucuronidase release. NaF (0.2 mM), like DFMO, inhibited calcitriol-stimulated calcium release without affecting medium beta-glucuronidase activity, whereas elevated phosphate (3 mM) inhibited both activities. The results suggest that DFMO, over the concentration range studied, inhibits calcium release by making the matrix resistant to resorption rather than by acting at a cellular locus.
Insights
Alpha-difluoromethylornithine (DFMO) inhibits bone resorption by making bone matrix resistant to breakdown, not by affecting osteoclast activation. This mechanism differs from polyamine depletion effects on bone metabolism.
Area of Science:
- Biochemistry
- Bone Biology
- Pharmacology
Background:
- Ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO) previously suggested to inhibit bone resorption via polyamine-independent mechanisms.
- Calcitriol is known to stimulate bone resorption, a process involving osteoclasts.
Purpose of the Study:
- To investigate whether DFMO prevents calcitriol-stimulated bone resorption by acting before or after osteoclast activation.
- To elucidate the specific mechanism by which DFMO affects bone resorption.
Main Methods:
- Cultured neonatal mouse calvaria were treated with DFMO, ornithine, putrescine, spermidine, spermine, NaF, or elevated phosphate.
- Calcium and beta-glucuronidase release into the culture medium were measured as indicators of bone resorption and osteoclast activity, respectively.
Main Results:
- DFMO inhibited calcium release from calcitriol-stimulated calvaria but did not affect the increase in beta-glucuronidase secretion.
- Ornithine and other polyamines inhibited both calcium and beta-glucuronidase release.
- NaF mimicked DFMO's effect by inhibiting calcium release without altering beta-glucuronidase, while elevated phosphate inhibited both.
Conclusions:
- DFMO inhibits calcium release by increasing bone matrix resistance to resorption, rather than by acting on osteoclast cellular activity.
- The mechanism of DFMO action on bone resorption is distinct from polyamine-mediated effects and NaF's action.
- DFMO's effect appears to be matrix-dependent, suggesting a novel therapeutic target for bone resorption disorders.