MCP-1 expression is specifically regulated during activation of skeletal repair and remodeling

Monocyte chemotactic protein-1 (MCP-1) belongs to the CC chemokine superfamily and plays a critical role in the recruitment and activation of leukocytes during acute inflammation. We hypothesize that MCP-1 is also an important chemokine that regulates the recruitment and activation of bone cells required for skeletal repair and remodeling. We used the ulnar stress fracture (SFx) model, which allows investigation of focal remodeling with a known time course and precise anatomical location. SFx were created in the right ulna of female Wistar rats using cyclic end loading. Unloaded animals were used as a control. Rats were killed 4 h and 1, 4, 7, and 14 days after loading (n = 10/group); RNA was extracted and converted to cDNA for quantitative PCR analysis using TaqMan gene expression assays. Four hours after loading, MCP-1 gene expression was increased ~30-fold (P < 0.001), remained elevated at 24 h (~12-fold, P < 0.001), then declined by day 14. Relative to the contralateral limb, expression of the receptors CCR1 and CCR2 increased over the 14 days, being significant by 4 days for CCR1 and 14 days for CCR2 (P < 0.05). Other inflammation-related chemokines (RANTES, MIP1a) were not increased at these early time points. Using in situ hybridization and immunohistochemistry in separate animal groups (n = 5/group, control, days 1, 4, 7), MCP-1 mRNA and protein were localized in periosteal osteoblasts associated with woven bone formation at the fracture exit point but not in osteocytes adjacent to the SFx. These data support an important role for MCP-1 in the early phase of SFx repair and activated remodeling.