p21 Inhibition Rescues Radiation-Induced Osteogenic Dysfunction and Promotes Bone Regeneration in Osteoradionecrosis of the Jaw via the Rb–Smad2/3–Runx2 Axis

Background Osteoradionecrosis of the jaw (ORNJ) is a severe radiotherapy complication with unclear molecular drivers. CDKN1A/p21 is upregulated by radiation-induced DNA damage, but its role in ORNJ remains unknown. Methods An ORNJ mouse model was established. Single-cell RNA-sequencing of irradiated mandibles was analyzed and validated in vivo. Irradiated jaw bone marrow mesenchymal stem cells (JBMMSCs) were characterized. p21 was inhibited using UC2288 and siRNA. A UC2288-loaded AlgMA hydrogel was developed for local delivery. Results Bone healing was compromised in ORNJ. Radiation induced p21 upregulation in osteogenic lineage cells, which promoted Rb dephosphorylation and aberrant Smad2/3 phosphorylation, thereby suppressing Runx2-mediated osteogenesis. p21 inhibition reversed this cascade and restored osteogenic differentiation. Local delivery of UC2288@AlgMA significantly enhanced bone regeneration. Conclusion p21-related signaling impairs bone healing after irradiation, and local UC2288 delivery via AlgMA hydrogels represents a viable strategy to enhance ORNJ regeneration.