Adrenomedullin 2 improves bone regeneration in type 1 diabetic rats by restoring imbalanced macrophage polarization and impaired osteogenesis

Background Both advanced glycation end products (AGEs) and AGE-mediated M1 macrophage polarization contribute to bone marrow mesenchymal stem cell (BMSC) dysfunction, leading to impaired bone regeneration in type 1 diabetes mellitus (T1DM). Adrenomedullin 2 (ADM2), an endogenous bioactive peptide belonging to the calcitonin gene-related peptide family, exhibits various biological activities associated with the inhibition of inflammation and reduction of insulin resistance. However, the effects and underlying mechanisms of ADM2 in AGE-induced macrophage M1 polarization, BMSC dysfunction, and impaired bone regeneration remain poorly understood. Methods The polarization of bone marrow-derived macrophages was verified using flow cytometry analysis. Alkaline phosphatase (ALP) staining, ALP activity detection, and alizarin red staining were performed to assess the osteogenesis of BMSCs. Quantitative real-time polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting, and immunofluorescence staining were used to assess polarization markers, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling, and osteogenic markers. In vivo, a distraction osteogenesis (DO) rat model with T1DM was established, and tibia samples were collected at different time points for radiological, biomechanical, and histological analyses, to verify the effects of ADM2 on bone regeneration and M2 polarization under diabetic conditions. Results ADM2 treatment reversed AGE-induced M1 macrophage polarization towards the M2 phenotype, which was partially achieved by the peroxisome proliferator-activated receptor γ (PPARγ)-mediated inhibition of NF-κB signaling. The PPARγ inhibitor GW9662 significantly attenuated the effects of ADM2. Besides, ADM2 treatment improved the AGE-impaired osteogenic potential of BMSCs in vitro. Furthermore, ADM2 accelerated bone regeneration, as revealed by improved radiological and histological manifestations and biomechanical parameters, accompanied by improved M2 macrophage polarization in diabetic DO rats, and these effects were partially blocked by GW9662 administration. Conclusions These results indicate that ADM2 enhances diabetic bone regeneration during DO, by attenuating AGE-induced imbalances in macrophage polarization, partly through PPARγ/NF-κB signaling, and improving AGE-impaired osteogenic differentiation of BMSCs simultaneously. These findings reveal that ADM2 may serve as a potential bioactive factor for promoting bone regeneration under diabetic conditions, and imply that management of inflammation and osteogenesis, in parallel, may present a promising therapeutic strategy for diabetic patients during DO treatment.