Radiotherapy is increasingly used in the treatment of
joint diseases, but limited information is available on the effects of radiation on cartilage. Here, we characterize the molecular mechanisms leading to cellular senescence in irradiated primary cultured articular chondrocytes. Ionizing radiation (IR) causes activation of ERK, in turn generating intracellular
reactive oxygen species (ROS) with induction of senescence-associated
beta-galactosidase (SA-beta-gal) activity. ROS activate p38
kinase, which further promotes ROS generation, forming a positive feedback loop to sustain ROS-p38
kinase signaling. The ROS inhibitors,
nordihydroguaiaretic acid and GSH, suppress phosphorylation of p38 and cell numbers positive for SA-beta-gal following irradiation. Moreover, inhibition of the ERK and p38 kinase pathways leads to blockage of IR-induced SA-beta-gal activity via reduction of ROS generation. Although JNK is activated by ROS, this pathway is not associated with cellular senescence of chondrocytes. Interestingly, IR triggers down-regulation of
SIRT1 protein expression but not the transcript level, indicative of post-transcriptional cleavage of the
protein.
SIRT1 degradation is markedly blocked by SB203589 or
MG132 after IR treatment, suggesting that cleavage occurs as a result of binding with p38
kinase, followed by processing via the 26 S proteasomal degradation pathway. Overexpression or activation of
SIRT1 significantly reduces the IR-induced senescence phenotype, whereas inhibition of
SIRT1 activity induces senescence. Based on these findings, we propose that IR induces cellular senescence of articular chondrocytes by negative post-translational regulation of
SIRT1 via ROS-dependent p38
kinase activation.