Transferrin promotes chondrogenic differentiation in condylar growth through inducing autophagy via ULK1-ATG16L1 axis

Skeletal mandibular hypoplasia (SMH) is one of the most common skeletal craniofacial deformities in orthodontics, which was often accompanied by impaired chondrogenesis and increasing apoptosis of condylar chondrocytes. Therefore, protecting chondrocytes from apoptosis and promoting chondrogenesis in condylar growth is vital for treatment of SMH patients. Transferrin (TF) was highly expressed in condylar cartilage of newborn mice and was gradually declined as the condyle ceased growing. Interestingly, serum level of TF in SMH patients was significantly lower than normal subjects. Hence, the aim of our study was to investigate the effect of TF on survival and differentiation of chondrocytes and condylar growth. First, we found that TF protected chondrogenic cell line ATDC5 cells from hypoxia-induced apoptosis and promoted proliferation and chondrogenic differentiation in vitro. Second, TF promoted chondrogenic differentiation and survival through activating autophagic flux. Inhibiting autophagic flux markedly blocked the effects of TF. Third, TF significantly activated ULK1-ATG16L1 axis. Silencing either transferrin receptor (TFRC), ULK1/2 or ATG16 significantly blocked the autophagic flux induced by TF, as well as its effect on anti-apoptosis and chondrogenic differentiation. Furthermore, we established an organoid culture model of mandible ex vivo and found that TF significantly promoted condylar growth. Taken together, our study unraveled a novel function of TF in condylar growth that TF protected chondrocytes from hypoxia-induced apoptosis and promoted chondrogenic differentiation through inducing autophagy via ULK1-ATG16L1 axis, which demonstrated that TF could be a novel growth factor of condylar growth and shed new light on developing treatment strategy of SMH patients.