Mechanical stress is thought to be involved in the pathogenesis and pathophysiology of osteoarthritis (OA). However, much remains to be elucidated regarding how chondrocytes sense and respond to mechanical stress (stress sensing and response factors). Additionally, it still remains unclear whether there are defensive responses and mechanisms to protect against pathological agents and mechanical stress in articular cartilage tissue. This study was designed to determine whether repetitive mechanical force, at physiologic levels, affects the expression of factors regulating autophagy such as the autophagy-related proteins ATG5, Beclin-1, and Parkin, and the autophagy process as well as cellular activity in cultured chondrocytes. Three-dimensional cultured tissue was generated from human chondrocytes using a collagen sponge scaffold. After physiological mechanical loading of the 3D cell-collagen sponge construct, comparative analyses of expression levels of ATG5, Beclin-1, and Parkin were performed in human chondrocytes. Chondrocyte activity and Transmission Electron Microscopy (TEM) analysis for detecting autophagy process were also analyzed with or without repetitive compressive loading. In chondrocytes, 60 min or 180 min repetitive compressive loading significantly decreased the expression of ATG5, Beclin-1 and Parkin in comparison with the non-loading group. TEM analysis indicated that, in normal chondrocytes of the non-loading group, the autophagy process was shown to be progressing. In contrast, repetitive loading decreased the number of autophagosomes and autolysosomes in chondrocytes. In addition, numerous degenerated organelles that had not undergone autophagy were observed within the chondrocytes under repetitive loading. The ATG5 and Beclin-1 proteins are known to play crucial roles in regulating cellular autophagy. Furthermore, repetitive mechanical loading caused a decreased expression of Parkin, a mitophagy regulator in chondrocytes. Our results indicate for the first time that a decrease in mitophagy, as well as cellular autophagy, in response to mechanical stress, even at the physiologic level, leads to the accumulation of defective mitochondria and abnormal cellular proteins, resulting in reduced chondrocyte activity and affecting the maintenance of cartilage tissue homeostasis, ultimately contributing to the progression of OA.
