KMUP-1 Suppresses RANKL-Induced Osteoclastogenesis and Prevents Ovariectomy-Induced Bone Loss: Roles of MAPKs, Akt, NF-κB and Calcium/Calcineurin/NFATc1 Pathways

<div><p>Background</p><p>KMUP-1 is a xanthine derivative with inhibitory activities on the phosphodiesterase (PDE) 3,4 and 5 isoenzymes to suppress the degradation of cyclic AMP and cyclic GMP. However, the effects of KMUP-1 on osteoclast differentiation are still unclear. In this study, we investigated whether KMUP-1 inhibits osteoclastogenesis induced by RANKL in RAW 264.7 cells and bone loss induced by ovariectomy in mice, and the underlying mechanisms.</p><p>Principal Findings</p><p><i>In vitro</i>, KMUP-1 inhibited RANKL-induced TRAP activity, the formation of multinucleated osteoclasts and resorption-pit formation. It also inhibited key mediators of osteoclastogenesis including IL-1β, IL-6, TNF-α and HMGB1. In addition, KMUP-1 inhibited RANKL-induced activation of signaling molecules (Akt, MAPKs, calcium and NF-κB), mRNA expression of osteoclastogensis-associated genes (TRAP, MMP-9, Fra-1, and cathepsin K) and transcription factors (c-Fos and NFATc1). Furthermore, most inhibitory effects of KMUP-1 on RANKL-mediated signal activations were reversed by a protein kinase A inhibitor (H89) and a protein kinase G inhibitor (KT5823). <i>In vivo</i>, KMUP-1 prevented loss of bone mineral content, preserved serum alkaline phosphate and reduced serum osteocalcin in ovariectomized mice.</p><p>Conclusions</p><p>KMUP-1 inhibits RANKL-induced osteoclastogenesis <i>in vitro</i> and protects against ovariectomy-induced bone loss <i>in vivo</i>. These effects are mediated, at least in part, by cAMP and cGMP pathways. Therefore, KMUP-1 may have a role in pharmacologic therapy of osteoporosis.</p>