Reduced response of human meniscal cells to Osteogenic Protein 1 during osteoarthritis and pro-inflammatory stimulation

Objective: Many cell types lose responsiveness to anabolic factors during inflammation and disease. Osteogenic Protein 1 (OP1/BMP7) was evaluated for the ability to enhance extracellular matrix synthesis in healthy and OA meniscus cells. Mechanisms of cell response to OP1 were explored. Design: Meniscus and cartilage tissues from healthy tissue donors and osteoarthritis (OA) patients undergoing total knee arthroplasties were acquired. Primary cell cultures were stimulated with OP1 and/or inflammatory factors (IL1α, IL1β, or fibronectin fragments (FnF)) and cellular responses were analyzed by RT-qPCR and immunoblots. Frozen section immunohistochemistry (IHC) was conducted to assess OP1 and receptor proteins in normal and OA meniscus. Results: OP1 treatment of normal meniscus cells resulted in significant, dose-dependent increases in ACAN (aggrecan) and COL2A1, and decreased MMP13 gene transcription, while only ACAN was upregulated (P < 0.01) at the highest dose of OP1 in OA meniscus cells. OP1 induced significantly more ACAN gene transcription in normal meniscus than normal articular cartilage (P = 0.05), and no differences between normal and OA cartilage were detected. Receptor expression and kinetics of canonical signaling activation were similar between normal and OA specimens. Normal meniscus cells treated with inflammatory factors were refractory to OP1 stimulation. Smad1 phosphorylation at an inhibitory site was induced (P = 0.01 for both normal and OA meniscus) by inflammatory cytokine treatment. Conclusions: The meniscus demonstrates resistance to OP1 stimulation in OA and in the presence of inflammatory mediators. MAPK-mediated Smad1 linker phosphorylation is a possible mediator of the loss of anabolic extracellular matrix production in the inflammatory cytokine affected meniscus.