NAD-dependent deacetylase Sirtuin-1 down-regulates expressions of osteogenic transcription facter RUNX2 and matrix metaprotinase (MMP)-13 in osteoarthritic chondrocytes

Purpose: During the development of osteoarthritis (OA), mechanical force to articular cartilage and inflammation downregulate the anabolic activities of chondrocytes and accelerate the cartilage degeneration. Recently, several reports have demonstrated that NAD-dependent deacetylase sirtuin-1 (SIRT1) plays important roles in a variety of biological processes such as DNA repair, energy metabolism and gene expressions. It has been reported that SIRT1 insufficiency induces the vascular calcification and artherosclerosis in cardiovascular tissues, suggesting that SIRT1 may be of benefit in the protection against vascular calcification and maintenance of cardiovascular function. More recently, it has been suggested that SIRT1 promotes osteogenic and chondrogenic differentiation of mesenchymal stem cells and SIRT1 inhibits apoptosis of human chondrocytes. However, the role of SIRT1 in the osteoarthritic chondrocyte state still remains unclear. It is well known that SIRT1 activity is inhibited by aging and cellular stresses such as inflammation, oxidative stress and mechanical stress. We postulated that SIRT1 has a potential to down-regulate a hypertrophic chondrocyte lineage and OA progression through the inactivation of Runt-related transcription factor 2 (RUNX2) and matrix metalloproteinases (MMPs) in osteoarthritic chondrocytes. To clarify whether SIRT1 regulates chondrocyte activity in OA, we investigated expressions of SIRT1, RUNX2 and MMP-13, and their correlations in human osteoarthritic chondrocytes. Methods: Human chondrocytes were isolated from articular cartilage tissues from four patients with knee OA (66, 74, 77, 78 years old) and one patient (75 years old) with idiopathic osteonecrosis of the knee, who underwent the knee joint replacement surgery. Levels of RUNX2, SIRT1 and MMP-13 expressions were analyzed in the presence or absence of OA relating catabolic factor, IL-1beta 10.0 ng/ml, in human osteoarthritic chondrocytes. The effect of SIRT1 inactivation on chondrocyte activity was also examined in OA chondrocytes cultures that were treated with SIRT1 inhibitor (S)-35 (1.0 microM, Santa Cruz Biotechnologies,Santa Cruz,CA). Results: RUNX2 was ubiquitously expressed in osteoarthritic chondrocytes from OA patients, but not in chondrocytes from patient with idiopathic osteonecrosis of the knee. IL-1beta significantly up-regulated the expression of MMP-13 in osteoarthritic chondrocytes. Osteoarthritic chondrocytes that were treated with SIRT1 inhibitor (S)-35 expressed a higher level of RUNX2 in comparison with the control. IL-1beta-induced production of MMP-13 from osteoarthritic chondrocytes was accelerated by the pretreatment with SIRT1 inhibitor (S)-35 in a dose dependent manner. Conclusions: Our study indicated that SIRT1 inactivation induced the marked expression of hypertrophic marker of chondrocytes, RUNX2, and the production of matrix degrading enzyme MMP-13 in OA chondrocytes. These findings suggest that NAD-dependent deacetylase SIRT1 may prevent a hypertrophy of chondrocytes and cartilage degradation during the progression of OA. Since it is well known that SIRT1 activity is downregulated by cellular stresses involving OA-relating stresses, such as inflammation and oxidative stress, as well as aging, SIRT inactivation in chondrocytes may be involved in the pathogenesis of OA.