Hypericin suppresses osteoclast formation and wear particle-induced osteolysis via modulating ERK signalling pathway

Osteoclast-induced bone resorption and wear-particle-induced osteolysis leads to prosthetic loosening, one of the most common causes of joint implant failure, resulting in revision surgery. Thus, inhibition of osteoclastic bone resorption, which further prevents wear particle-induced osteolysis, is a potential treatment strategy for prosthetic loosening. Here, we examined the therapeutic effect of hypericin (HP), which was photosensitive, on osteoclastogenesis and wear particle-induced osteolysis in the absence of visible light. HP inhibited RANKL-induced osteoclast differentiation in bone marrow macrophages (BMMs) and RAW264.7 cell line without any evidence of cytotoxicity. The bone-resorbing activity of mature osteoclasts was significantly inhibited by HP. As HP has been previously reported to inhibit signalling pathway such as ERK and NF-κB in other cells, which is also important in osteoclast differentiation. We thus examined the molecular mechanism and showed that HP significantly inhibited the ERK/mitogen-activated protein kinase (MAPK) signalling pathway without affecting nuclear factor kappaB (NF-κB), c-Jun N-terminal kinase (JNK) and p38 signalling in RANKL-stimulated BMMs. Further in vivo studies revealed HP attenuated osteoclast formation and subsequently prevented wear particle-induced bone erosion. Taken together, the results suggest that HP inhibits RANKL-mediated osteoclastogenesis via affecting ERK signalling in vitro and suppresses wear particle-induced osteolysis in vivo. We therefore conclude that HP may be an innovative and safe alternative treatment for osteoclast-related prosthetic loosening.