The HH Gli pathway has been demonstrated to play
The HH-Gli pathway has been demonstrated to play an extensive role in skeletal development. India Hedgehog, derived from hypertrophic chondrocytes, and Parathyroid Hormone-like related Protein (PTHrP), derived from peri-articular chondrocytes, form a negative feedback loop to inhibit chondrocyte hypertrophy (Lanske et al., 1996). Additionally, the HH-Gli pathway controls chondrocyte proliferation independent of PTHrP (Koziel et al., 2004). Additional data have suggested that the HH-Gli pathway is also involved in osteoblast differentiation (Hojo et al., 2012, Jemtland et al., 2003, Ohba et al., 2008, van der Horst et al., 2003). Furthermore, HH-Gli signaling is involved in bone marrow macrophage (BMM) proliferation and osteoclast formation in the presence of metastatic tumors (Das et al., 2011, Heller et al., 2012). However, it remains unknown whether BMM proliferation anticipates the osteoclastogenesis and osteoclast apoptosis observed in postmenopausal osteoporosis. The molecular mechanism underlying the interaction between 3280 and Hedgehog signaling also remains unclear.
ERK1 and ERK2, which can be activated by mitogen-activated protein kinase (MEK), phosphorylate a series of substrates and control cell proliferation and differentiation (Ahearn et al., 2012, Chung and Kondo, 2011, Pylayeva-Gupta et al., 2011, Ward et al., 2012). Estrogen has been suggested to promote the activation of MEK/ERK signaling in osteoclasts (Chen et al., 2005, Xie et al., 2011). Yang reported that Ihh and PTHrP were increased under MEK inhibitor treatment (Yang et al., 2013); however, it has not yet been determined whether the increased and activated osteoclasts in postmenopausal osteoporosis are regulated by MEK/ERK via the activation of HH-Gli signaling.
Materials and methods
Discussion Postmenopausal osteoporosis is a common and complicated health problem that endangers postmenopausal women in all countries. Estrogen deficiency triggers enhanced bone resorption. This osteoclast-mediated process is believed to be the primary driving factor. Based on this, estrogen therapy and selective estrogen receptor modulators have been developed as effective options for preventing and treating postmenopausal osteoporosis (Montagnani, 2014). However, the side effects of estrogen therapy limit its applications. Although many other anti-osteoporosis drugs have been approved for clinical applications, there is still an urgent need for new and effective treatment methods. Because postmenopausal osteoporosis is characterized by over-activated osteoclast-mediated bone resorption, dysregulated osteoclasts should be focused on to identify treatment methods in postmenopausal osteoporosis. Osteoclasts are derived from circulating monocytes which migrate to bone and differentiate to mature osteoclasts there (Hwang et al., 2012). Both previous reports and our data have confirmed that osteoclast proliferation and differentiation are inhibited by estrogen, but that osteoclast apoptosis is promoted by estrogen (Kimble et al., 1996, Martin and Udagawa, 1998). Das et al. reported that metastatic tumors of the bone secrete Hedgehog ligands and induce pathological bone resorption and fracture (Das et al., 2011). Additionally, the Hedgehog signaling pathway promotes bone resorption by inducing macrophage proliferation and osteoclast differentiation (Heller et al., 2012). Therefore, we speculated that Hedgehog signaling plays an essential role in upregulating osteoclasts when estrogen is insufficient. According to Robbins’ definition, Hedgehog signaling can be divided into canonical and noncanonical types pathways according to the participation of Gli transcription factors (Robbins et al., 2012). Because the HH-Gli pathway (the canonical Hedgehog signaling pathway) is involved in a large variety of tissues and cell behaviors, we studied this type first. We found that the HH-Gli pathway was activated in osteoclasts derived from estrogen-deficient OVX mice, indicating a negative relationship between estrogen and the HH-Gli pathway. Then, we confirmed these results in osteoclasts derived from RAW264.7 cells and had similar observations.