SirT1 enhances survival of human osteoarthritic chondrocytes by repressing protein tyrosine phosphatase 1B and activating the insulin-like growth factor receptor pathway.
OBJECTIVE
The protein deacetylase SirT1 inhibits apoptosis in a variety of cell systems by distinct mechanisms, yet its role in chondrocyte death has not been explored. We undertook the present study to assess the role of SirT1 in the survival of osteoarthritic (OA) chondrocytes in humans.


METHODS
SirT1, protein tyrosine phosphatase 1B (PTP1B), and PTP1B mutant expression plasmids as well as SirT1 small interfering RNA (siRNA) and PTP1B siRNA were transfected into primary human chondrocytes. Levels of apoptosis were determined using flow cytometry, and activation of components of the insulin-like growth factor receptor (IGFR)/Akt pathway was assessed using immunoblotting. OA and normal knee cartilage samples were subjected to immunohistochemical analysis.


RESULTS
Expression of SirT1 in chondrocytes led to increased chondrocyte survival in either the presence or the absence of tumor necrosis factor alpha/actinomycin D, while a reduction of SirT1 by siRNA led to increased chondrocyte apoptosis. Expression of SirT1 in chondrocytes led to activation of IGFR and the downstream kinases phosphatidylinositol 3-kinase, phosphoinosite-dependent protein kinase 1, mTOR, and Akt, which in turn phosphorylated MDM2, inhibited p53, and blocked apoptosis. Activation of IGFR occurs at least in part via SirT1-mediated repression of PTP1B. Expression of PTP1B in chondrocytes increased apoptosis and reduced IGFR phosphorylation, while down-regulation of PTP1B by siRNA significantly decreased apoptosis. Examination of cartilage from normal donors and OA patients revealed that PTP1B levels are elevated in OA cartilage in which SirT1 levels are decreased.


CONCLUSIONS
For the first time, it has been demonstrated that SirT1 is a mediator of human chondrocyte survival via down-regulation of PTP1B, a potent proapoptotic protein that is elevated in OA cartilage.
National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH, Bethesda, Maryland 20892, USA.
Viktoria
V
Gagarina
Odile
O
Gabay
Mona
M
Dvir-Ginzberg
Eun Jin
EJ
Lee
Jillian K
JK
Brady
Michael J
MJ
Quon
David J
DJ
Hall
D000368:Aged; D017209:Apoptosis / Q000502:physiology*; D002358:Cartilage, Articular / Q000378:metabolism / Q000473:pathology; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000378:metabolism / Q000473:pathology*; D015536:Down-Regulation / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D010003:Osteoarthritis / Q000378:metabolism / Q000473:pathology*; D010766:Phosphorylation / Q000502:physiology; D054562:Protein Tyrosine Phosphatase, Non-Receptor Type 1 / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051736:Proto-Oncogene Proteins c-mdm2 / Q000378:metabolism; D017451:Receptors, Somatomedin / Q000378:metabolism*; D056564:Sirtuin 1 / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
D017451:Receptors, Somatomedin; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2; D051057:Proto-Oncogene Proteins c-akt; C516607:PTPN1 protein, human; D054562:Protein Tyrosine Phosphatase, Non-Receptor Type 1; C447939:SIRT1 protein, human; D056564:Sirtuin 1