The pathology of oxidative stress-induced autophagy in a chronic rotator cuff enthesis tear

Abstract

Partial-thickness rotator cuff tears (PTRCTs) are often found in daily orthopedic practice, with most of the tears occurring in middle-aged patients. An anaerobic process and imbalanced oxygenation have been observed in PTRCTs, resulting in oxidative stress. Studies have shown the roles of oxidative stress in autophagy and the potential of unregulated mechanisms causing disturbance in soft tissue healing. This article aims to review literature works and summarize the potential pathology of oxidative stress and unregulated autophagy in the rotator cuff enthesis correlated with chronicity. We collected and reviewed the literature using appropriate keywords, in addition to the manually retrieved literature. Autophagy is a normal mechanism of tissue repair or conversion to energy needed for the repair of rotator cuff tears. However, excessive mechanisms will degenerate the tendon, resulting in an abnormal state. Chronic overloading of the enthesis in PTRCTs and the hypovascular nature of the proximal tendon insertion will lead to hypoxia. The hypoxia state results in oxidative stress. An autophagy mechanism is induced in hypoxia via hypoxia-inducible factors (HIFs) 1/Bcl-2 adenovirus E1B 19-kDa interacting protein (BNIP) 3, releasing beclin-1, which results in autophagy induction. Reactive oxygen species (ROS) accumulation would induce autophagy as the regulator of cell oxidation. Oxidative stress will also remove the mammalian target of rapamycin (mTOR) from the induction complex, causing phosphorylation and initiating autophagy. Hypoxia and endoplasmic reticulum (ER) stress would initiate unfolded protein response (UPR) through protein kinase RNA-like ER kinase (PERK) and activate transcription factor 4, which induces autophagy. Oxidative stress occurring in the hypovascularized chronic rotator cuff tear due to hypoxia and ROS accumulation would result in unregulated autophagy directly or autophagy mediated by HIF-1, mTOR, and UPR. These mechanisms would disrupt enthesis healing.

Hypoxia-Inducible Factor and Oxidative Stress in Tendon Degeneration: A Molecular Perspective

“…Oxidative stress, caused by an imbalance between ROS and antioxidants, is another physiological condition implicated in various musculoskeletal disorders [15]. Tendon injury leads to a state of oxidative stress, marked by an increase in ROS species, which upregulate pro-inflammatory cytokines that promote apoptosis and extracellular matrix degradation [19]. Antioxidants such as Vitamin C, Curcumin, Nicotinamide Mononucleotide (NMN), and Superoxide Dismutase (SOD) [20][21][22][23] have been reported to promote tendon repair, but their exact mode of action requires further exploration [20].…”