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Chondrocyte apoptosis plays an important role in cartilage degeneration in osteoarthritis (OA), and mechanical injury to cartilage induces chondrocyte apoptosis. In response to DNA damage, p53 expression is up-regulated, transcription activity is increased, and apoptosis signals are initiated. The p53-regulated apoptosis-inducing protein 1 (p53AIP-1) is one of the p53-regulated genes, and is activated in response to DNA damage. This study was undertaken to analyze p53 function after induction of apoptosis by shear strain in chondrocytes.
OA cartilage samples were obtained from subjects undergoing total knee replacement surgery, and normal cartilage samples were obtained from subjects undergoing surgery for femoral neck fracture. Chondrocytes were isolated from human cartilage and cultured. Expression of p53 and p53AIP in chondrocytes was detected by reverse transcriptase-polymerase chain reaction and Western blotting. Shear strain was introduced in normal human knee chondrocytes. To explore p53 function, normal human knee chondrocytes were pretreated with pifithrin-alpha or p53 small interfering RNA (siRNA) before induction of shear strain. Chondrocyte apoptosis was detected by expression of cleaved caspase 9 with Western blotting and TUNEL staining. Expression of p53 and p53AIP-1 was analyzed by Western blotting.
OA and normal chondrocytes expressed p53. OA chondrocytes showed much higher expression of p53 and p53AIP-1 than did normal chondrocytes. TUNEL-positive cells and expression of p53, p53AIP-1, and cleaved caspase 9 were increased by shear strain, but chondrocyte apoptosis was suppressed after pretreatment with pifithrin-alpha or p53 siRNA.
Our findings indicate that p53 and p53AIP-1 play important roles in human chondrocyte apoptosis. Down-regulation of p53 expression prevents cartilage from undergoing apoptosis introduced by shear strain.