Osteoblastic monocyte chemoattractant protein-1 (MCP-1) mediation of parathyroid hormone's anabolic actions in bone implicates TGF-β signaling

Parathyroid hormone (PTH) is necessary for the regulation of calcium homeostasis and PTH (1-34) was the first approved osteoanabolic therapy for osteoporosis. It is well established that intermittent PTH increases bone formation and that bone remodeling and several cytokines and chemokines play an essential role in this process. Earlier, we had established that the chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2), was the most highly stimulated gene in rat bone after intermittent PTH injections. Nevertheless, MCP-1 function in bone appears to be complicated. To identify the primary cells expressing MCP-1 in response to PTH, we performed in situ hybridization of rat bone sections after hPTH (1-34) injections and showed that bone-lining osteoblasts are the primary cells that express MCP-1 after PTH treatment. We previously demonstrated MCP-1's importance by showing that PTH's anabolic effects are abolished in MCP-1 null mice, further implicating a role for the chemokine in this process. To establish whether rhMCP-1 peptide treatment could rescue the anabolic effect of PTH in MCP-1 null mice, we treated 4-month-old wild-type (WT) mice with hPTH (1-34) and MCP-1^-/- mice with rhMCP-1 and/or hPTH (1-34) for 6 weeks. Micro-computed tomography (μCT) analysis of trabecular and cortical bone showed that MCP-1 injections for 6 weeks rescued the PTH anabolic effect in MCP-1^-/- mice. In fact, the combination of rhMCP-1 and hPTH (1-34) has a synergistic anabolic effect compared with monotherapies. Mechanistically, PTH-enhanced transforming growth factor-β (TGF-β) signaling is abolished in the absence of MCP-1, while MCP-1 peptide treatment restores TGF-β signaling in the bone marrow. Here, we have shown that PTH regulates the transcription of the chemokine MCP-1 in osteoblasts and determined how MCP-1 affects bone cell function in PTH's anabolic actions. Taken together, our current work indicates that intermittent PTH stimulates osteoblastic secretion of MCP-1, which leads to increased TGF-β signaling, implicating it in PTH's anabolic actions.