NAD-dependent deacetylase sirtuin-1 mediates the mechanical stress-induced degeneration of articular cartilage in osteoarthritis: mechanical stress stimulates the expression of osteogenic transcription factor RUNX2 via the srituin-1 activation in chondrocytes

Abstract

Purpose: Mechanical stress is the major pathologic factors associated with osteoarthritis (OA). It has been indicated that the NAD-dependent deacetylase sirtuin-1 (Sirt-1) has an important role in human aging. Recent report demonstrated that SIRT-1 insufficiency induced the shear (mechanical) stress-induced vascular calcification and artherosclerosis in cardiovascular tissues, suggesting that SIRT-1 may be of benefit in the protection against vascular calcification and maintenance of cardiovascular function against shear stress. In addition, it has been suggested that Sirt-1 promotes osteogenic and chondrogenic differentiation of mesenchymal stem cells. However, the impact of Sirt-1 in the osteoarthritic chondrocyte state still remains unclear. We postulated that Sirt-1 regulates a mechanical stress-induced hypertrophic chondrocyte lineage and osteophyte formation through the activation of osteogenic transcription factor Runx2 in osteoarthritic chondrocytes. Since it is well known that Runx2 is a promotor of the activation of cartilage matrix degrading enzyme, matrix metalloproteinase (MMP)-13, Sirt-1 may also influence the expression of MMP-13 in chondrocytes, consequently facilitating the deterioration of articular cartilage in OA. To verify the impact of Sirt-1 in the pathology of OA, we investigated the mechanical stress-induced expressions of Sirt-1, Runx2 and MMP-13, and their correlations in human chondrocytes in OA. Methods: i) The expressions of Sirt-1 and Runx2 were histologically analyzed in the in vitro OA model (mechanical stress-culture system). ii) Human chondrocytes were isolated from articular cartilage tissues from OA patients who underwent the knee joint replacement surgery. Levels of expressions of Runx2, Sirt-1 and MMP-13 were analyzed in the presence or absence of mechanical stress (25 gf/cm2, 2.5 kPa), in chondrocytes. The impact of Sirt-1 insufficiency in the chondrocyte activity was also examined in chondrocytes cultures that were treated with Sirt-1 inhibitor. Results: i) The expression of Sirt-1 was ubiquitously observed in chondrocytes, in contrast, Runx2 expressed in the only osteophyte region in the knee joint of OA model mice. ii) Mechanical stress up-regulated the expression of Runx2 in osteoarthritic chondrocytes. The mechanical stress-accelerated expression of Runx2 was inhibited by the pretreatment with Sirt1 inhibitor in osteoarthritic chondrocytes. Also, the mechanical stress-induced production of MMP-13 from chondrocytes was reduced by the pretreatment with Sirt1 inhibitor. Conclusions: Our study revealed that Sirt-1 inactivation trended to inhibit the mechanical stress-induced expressions of Runx2 and MMP-13 in chondrocytes. The Runx2 has been reported to have an important role as a promotor of activation of MMP-13 in chondrocytes (4). Thus, our findings suggest that the NAD-dependent deacetylase Sirt-1 may upregulate the osteophyte formation and the expression of cartilage degrading enzyme MMP-13 through the mechanism involving the acceleration of the osteogenic transcription factor Runx2 in OA cartilage tissues. Since it is well known that Sirt-1 activity is affected by several stresses as well as mechanical stress, Sirt-1 may be involved in the pathology of OA. Our study may provide a pathologic mechanism linking the NAD-dependent deacetylase Sirt-1 and its modulation of Runx2 in OA.