Repetitive Compressive Loading Downregulates Mitochondria Function and Upregulates the Cartilage Matrix Degrading Enzyme MMP-13 Through the Coactivation of NAD-Dependent Sirtuin 1 and Runx2 in Osteoarthritic Chondrocytes.

Mechanical stress is known to be a pivotal risk factor in the development of OA. However, the involvement of repetitive compressive loading in mitochondrial dysfunction in chondrocytes remains unclear. The aim of this study was to investigate whether physiologic levels of repetitive mechanical force affect the regulation of energy metabolism and activities of mitochondrial function regulators, sirtuin 1 and nicotinamide adenine dinucleotide (NAD) in chondrocytes, and to clarify any correlation with chondrocyte catabolic activity. Repetitive physiological mechanical stress was applied in a 3D chondrocyte-collagen scaffold construct, and the 3D cultured tissues were collected at different time points by collagenase treatment to collect cellular proteins. Changes in chondrocyte activity (cell proliferation, MMP-13 production), energy metabolism regulator levels (sirtuin 1), mitochondrial function (ATP production, NAD level), and the expression level of the osteogenic and hypertrophic chondrogenic transcription factor, runt-related transcription factor 2 (Runx2), were measured. Treatment with repetitive compressive loading resulted in no significant change in the cell viability of chondrocytes. In the repetitive mechanical loading group, there were statistically significant increases in MMP-13 production and expression of both sirtuin 1 and Runx2 in chondrocytes relative to the non-loading control group. Furthermore, ATP production and NAD activity in mitochondria decreased in the repetitive mechanical loading group. Our present study reveals that in chondrocytes, repetitive compressive loading accelerated sirtuin activation, which requires and consumes NAD within mitochondria, leading to a decrease of NAD and ultimately in reduced mitochondrial ATP production. Additionally, since sirtuin 1 is known to positively regulate Runx2 activity in chondrocytes, the activation of sirtuin 1 by repetitive load stimulation may induce an increase in the expression of Runx2, which promotes the expression of MMP-13, and subsequently enhances MMP-13 production. Our findings indicate that repetitive compression loading-mediated mitochondrial dysfunction plays a pivotal role in the progression of OA, primarily by driving the downregulation of ATP production and promoting the expression of the matrix-degrading enzyme MMP-13.