Inhibitor of protein kinase N3 suppresses excessive bone resorption in ovariectomized mice

Introduction The long-term inhibition of bone resorption suppresses new bone formation because these processes are coupled during physiological bone remodeling. The development of anti-bone-resorbing agents that do not suppress bone formation is urgently needed. We previously demonstrated that Wnt5a-Ror2 signaling in mature osteoclasts promoted bone-resorbing activity through protein kinase N3 (Pkn3). The p38 MAPK inhibitor SB202190 reportedly inhibited Pkn3 with a low Ki value (0.004 μM). We herein examined the effects of SB202190 on osteoclast differentiation and function in vitro and in vivo. Materials and methods Bone marrow cells were cultured in the presence of M-csf and GST-Rankl to differentiate into multinucleated osteoclasts. Osteoclasts were treated with increasing concentrations of SB202190. For in vivo study, 10-week-old female mice were subjected to ovariectomy (OVX). OVX mice were intraperitoneally administered with a Pkn3 inhibitor at 2 mg/kg or vehicle for 4 weeks, and bone mass was analyzed by micro-CT. Results SB202190 suppressed the auto-phosphorylation of Pkn3 in osteoclast cultures. SB202190 significantly inhibited the formation of resorption pits in osteoclast cultures by suppressing actin ring formation. SB202190 reduced c-Src activity in osteoclast cultures without affecting the interaction between Pkn3 and c-Src. A treatment with SB202190 attenuated OVX-induced bone loss without affecting the number of osteoclasts or bone formation by osteoblasts. Conclusions Our results showed that Pkn3 has potential as a therapeutic target for bone loss due to increased bone resorption. SB202190 is promising as a lead compound for the development of novel anti-bone-resorbing agents.