Cyclin-dependent kinase inhibitor-1 deficiency enhances bone destruction in a mouse model of arthritis

Rheumatoid arthritis (RA) causes bone destruction by activating inflammatory cytokines and osteoclasts. Cyclin-dependent kinase inhibitor 1 (p21), a cell cycle regulator, may influence this process; however, its role remains unclear. Therefore, we investigated the effect and potential mechanisms of p21 deficiency in bone loss in a mouse model of arthritis. Collagen antibody-induced arthritis (CAIA) was established in p21 knockout (<i>p21</i> ^<i>-/-</i> ) and wild-type mice. Bone destruction was analyzed using histology, micro-computed tomography, and bone strength testing; osteoclast formation and activity were evaluated using tartrate-resistant acid phosphatase (TRAP) staining and immunohistochemistry for cathepsin K. The expression of inflammatory cytokines and osteoclast-related genes was examined using immunohistochemistry and real-time polymerase chain reaction, respectively. <i>p21</i> ^<i>-</i> <i>/</i> ^<i>-</i> mice exhibited greater bone destruction and lower bone strength than wild-type mice. Additionally, TRAP and cathepsin K staining revealed significantly higher osteoclast count in <i>p21</i> ^<i>-</i> <i>/</i> ^<i>-</i> mice. Interleukin (IL)-6, IL-1β, tumor necrosis factor-alpha (TNF-α), and phosphorylated signal transducer and activator of transcription 3 (STAT3) levels were considerably higher in bone tissues of <i>p21</i> ^<i>-</i> <i>/</i> ^<i>-</i> mice than in those of wild-type mice. In vitro osteoclast differentiation in bone marrow macrophages (BMMs) was examined after IL-6 stimulation; osteoclast differentiation and osteoclast marker gene expression were significantly enhanced in <i>p21</i> ^<i>-/-</i> BMMs. Western blotting confirmed increased STAT3 phosphorylation in <i>p21</i> ^<i>-/-</i> BMMs; IL-6 treatment further amplified osteoclastogenesis in <i>p21</i> ^<i>-/-</i> BMMs. In conclusion, p21 deficiency exacerbates bone destruction in arthritis by promoting osteoclast differentiation and inflammatory cytokine expression via the IL-6/STAT3 pathway. Targeting p21 may offer therapeutic potential for preventing arthritis-related bone loss, such as in RA.