title: Ionizing radiation induces cellular senescence of articular chondrocytes via negative regulation of SIRT1 by p38 kinase.
abstract: 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.
authors.affiliation: Division of Radiation Cancer Research, Korea Institute of Radiological and Medical Sciences, Seoul 139-706.
authors.forename: Eun-Hee
authors.initials: EH
authors.lastname: Hong
authors.forename: Su-Jae
authors.initials: SJ
authors.lastname: Lee
authors.forename: Jae-Sung
authors.initials: JS
authors.lastname: Kim
authors.forename: Kee-Ho
authors.initials: KH
authors.lastname: Lee
authors.forename: Hong-Duck
authors.initials: HD
authors.lastname: Um
authors.forename: Jae-Hong
authors.initials: JH
authors.lastname: Kim
authors.forename: Song-Ja
authors.initials: SJ
authors.lastname: Kim
authors.forename: Jong-Il
authors.initials: JI
authors.lastname: Kim
authors.forename: Sang-Gu
authors.initials: SG
authors.lastname: Hwang
mesh_terms: D000818:Animals; D002356:Cartilage / Q000378:metabolism*; D002478:Cells, Cultured; D016922:Cellular Senescence / Q000235:genetics / Q000528:radiation effects*; D019902:Chondrocytes / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000235:genetics / Q000378:metabolism; D004789:Enzyme Activation / Q000235:genetics / Q000528:radiation effects; D005720:Gamma Rays / Q000009:adverse effects*; D006801:Humans; D007592:Joint Diseases / Q000235:genetics / Q000378:metabolism / Q000532:radiotherapy; D007976:Leupeptins / Q000494:pharmacology; D048670:MAP Kinase Kinase 4 / Q000235:genetics / Q000378:metabolism; D020935:MAP Kinase Signaling System / Q000235:genetics / Q000528:radiation effects; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism; D061988:Proteasome Inhibitors; D011817:Rabbits; D017382:Reactive Oxygen Species / Q000378:metabolism; D056564:Sirtuin 1 / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation / Q000235:genetics / Q000528:radiation effects; D048051:p38 Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
chemical_list: D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; D061988:Proteasome Inhibitors; D017382:Reactive Oxygen Species; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease; C447939:SIRT1 protein, human; D056564:Sirtuin 1; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde