Anti-osteoporotic activity of a blocker of the ubiquitin-proteasome system, bortezomib, has known to be achieved by directly opposed action in increased bone formation by osteoblasts and in decreased bone destruction by osteoclasts. However, the mechanisms underlying the proteasome blocker inhibition of osteoclast differentiation and function are not fully understood. Here, we observed that proteasome inhibitors, such as MG132 and bortezomib, in osteoclasts accelerated the degradation of c-Fms, a cognate receptor of macrophage colony-stimulating factor (M-CSF), and did not affect the amount of receptor activator of nuclear factor kappa-B (RANK), a receptor of receptor activator of nuclear factor kappa-B ligand (RANKL). c-Fms degradation induced by proteasome inhibitors was controlled by the activation of p38/tumor necrosis factor-alpha converting enzyme (TACE)-mediated regulated intramembrane proteolysis (RIPping). This was validated through the restoration of c-Fms using specific inhibitors of p38 and TACE, and a stimulation of p38-dependent TACE. In addition, c-Fms degradation by proteasome inhibition completely blocked M-CSF-mediated intrinsic signalling and led to the suppression of osteoclast differentiation and bone resorption. In a mouse model with intraperitoneal administration of lipopolysaccharide (LPS) that stimulates osteoclast formation and leads to bone loss, proteasome blockers prevented LPS-induced inflammatory bone resorption due to a decrease in the number of c-Fms-positive osteoclasts. Our study showed that accelerating c-Fms proteolysis by proteasome inhibitors may be a therapeutic option for inflammation-induced bone loss.

Mollusks have served as important sources of human food and medicine for a long time. Raw Pisidium coreanum, a freshwater bivalve of the phylum Mollusca, is used in traditional therapies in parts of Asia. However, the therapeutic effects of Pisidium coreanum on bone diseases are not known. We investigated the functional roles of Pisidium coreanum in osteoporotic bone diseases. Pisidium coreanum inhibited the differentiation of bone marrow-derived monocytic cells into mature osteoclasts in vitro. The ovariectomized mice that received oral administration of Pisidium coreanum showed improvements in both trabecular and cortical bones. This preventive activity of Pisidium coreanum against bone loss was due to limited osteoclast maturation with reduced osteoclast surface extent in trabecular bone tissue. The formation of large multinucleated osteoclasts in vitro was significantly decreased in response to Pisidium coreanum, consistent with the reduced expression levels of osteoclast markers and fusion-related genes, such as NFATc1, p65, integrin αvβ3, DC-STAMP, OC-STAMP, Atp6v0d2, FAK, CD44, and MFR. These data suggest that Pisidium coreanum inhibits osteoclast differentiation by negatively regulating the fusion of mononuclear osteoclast precursors. Thus, our data demonstrate the ability of Pisidium coreanum to effectively prevent estrogen-deficient osteoporosis through inhibition of multinucleated osteoclast formation.