Tetracyclines induce apoptosis in osteoclasts

Chemically modified tetracyclines (CMTs) are thought to inhibit bone resorption through inhibition of matrix metalloproteinases. Here we report that some tetracyclines also induce apoptosis in rabbit osteoclasts and inhibit differentiation and activity of osteoclasts in murine osteoblast/marrow cocultures. Apoptosis of mature rabbit osteoclasts increased from 5.5 ± 1.4% (mean ± SD) in control cultures to 44.9 ± 6.3% (p< 0.001) and 18.9 ± 4.0% (p< 0.005) with CMT-3 and doxycycline (10 μg/mL), respectively. CMT-2 or CMT-5 did not alter osteoclast viability even at 25 μg/mL. In murine osteoblast/marrow cocultures over 11 days, CMT-3 and doxycycline (5 μg/mL) reduced the formation of mature osteoclasts and inhibited resorption to 21 ± 9% (p< 0.01) and 49 ± 4% (p< 0.01) of untreated cultures. Induction of osteoclast apoptosis is an additional property of tetracyclines that may contribute to their ability to inhibit bone resorption.