PTX3 promotes osteogenic differentiation by triggering HA/CD44/FAK/AKT positive feedback loop in an inflammatory environment

The treatment of periodontitis-induced alveolar bone defects remains a clinical challenge. The secreted protein pentraxin 3 (PTX3) protects tissue during inflammation and maintains bone homeostasis in physiological conditions. However, the effects of PTX3 on osteoblast differentiation and bone regeneration after periodontitis remain unclear. Here, we found that MC3T3-E1 mouse pre-osteoblast cells secreted increased PTX3 under TNF-α-induced inflammatory conditions in vitro. Gain-of-function and loss-of-function experiments revealed that PTX3 overexpression promoted osteogenic potential in an inflammatory environment and vice versa. The promoting effect was attributed to the regulatory role of PTX3 on the hyaluronan (HA)-dependent pericellular matrix (PCM). PTX3 was found in the HA-dependent PCM of MC3T3-E1 cells, where it promoted HA synthesis and the expression of CD44 (main HA receptor), enhancing the HA-CD44 interaction. The HA-CD44 interaction further activated focal adhesion kinase (FAK)/protein kinase B (AKT) signaling cascade. FAK/AKT activation promoted the expression of HA synthases 1/2/3 (HAS1/2/3) and CD44 in MC3T3-E1 cells under inflammatory condition, forming a positive feedback loop that activated by PTX3. Importantly, when HA was digested or any one of these molecules in the positive feedback loop was blocked, PTX3 partially lost the ability to promote osteogenic differentiation in an inflammatory environment. Ligatures were removed after seven days of periodontitis induction in vivo, to investigate alveolar bone regeneration after periodontitis. Histological and Micro-CT evaluation after seven and 14 days of local PTX3 treatment showed that alveolar bone healing was significantly improved compared to the vehicle control group. These findings suggested that PTX3 can induce osteogenic differentiation in an in vitro inflammatory environment by triggering the HA/CD44/FAK/AKT positive feedback loop, and promote bone regeneration after periodontitis.