15 Mechanical overloading is a major cause of tendinopathy, but the underlying pathogenesis of 16 tendinopathy is unclear. Here we report that high mobility group box1 (HMGB1) is released to 17 the tendon extracellular matrix and initiates an inflammatory cascade in response to mechanical 18 overloading in a mouse model. Moreover, administration of glycyrrhizin (GL), a naturally 19 occurring triterpene and a specific inhibitor of HMGB1, the tendon's inflammatory reactions.20 Also, while prolonged mechanical overloading in the form of long-term intensive treadmill 21 running induces Achilles tendinopathy in mice, administration of GL completely blocks the 22 tendinopathy development. Additionally, mechanical overloading of tendon cells in vitro induces 23 HMGB1 release to the extracellular milieu, thereby eliciting inflammatory and catabolic 24 responses as marked by increased production of prostaglandin E 2 (PGE 2 ) and matrix 25 metalloproteinase-3 (MMP-3) in tendon cells. Application of GL abolishes the cellular 26 inflammatory/catabolic responses. Collectively, these findings point to HMGB1 as a key 27 molecule that is responsible for the induction of tendinopathy due to mechanical overloading 28 placed on the tendon. 29 Keywords: Mechanical overloading, HMGB1, tendinopathy, tendon inflammation, tendon 30 degeneration, glycyrrhizin 31 32 Introduction 33 34Tendinopathy, a debilitating chronic tendon disorder, is manifested in clinical settings by 35 a combination of pain, swelling, compromised tendon structure, and rupture (1). Tendinopathy, 36 which involves tendon inflammation and degeneration, affects healthy individuals during their 37 active and productive years of life resulting in tremendous healthcare costs and economic impact 38 due to work-loss (2, 3). In particular, insertional tendinopathy, which is common in young 39 athletes, often occurs in the tendon proper proximal to the insertion into the heel bone and 40 accounts for about 20% of Achilles tendon disorders (4). It is well established that while normal 41 physiological loading is essential for tendon homeostasis, mechanical overloading induces the 42 development of tendinopathy, characterized by disorganized matrix, reduced numbers and 43 rounding of tendon cells, fibrocartilaginous change, and neovascularization (5, 6). A current 44 concept on the mechanisms of tendinopathy is that repetitive loading may lead to a 45 mechanobiological over-stimulation of tendon cells resulting in an imbalance between the 46 synthesis and breakdown of matrix proteins, especially collagen (7-9). The resulting mismatch is 47 a continuous loss of collagen in the tendon by repetitive loading with insufficient recovery time, 48 which initiates a catabolic degenerative response that leads to tendinopathy (6, 10).
50It is now recognized that inflammation is part of tendinopathy and could lead to tendon 51 degeneration that occurs at late stages of tendinopathy (11)(12)(13). Under excessive mechanical 52 loading, abnormal levels of proinflammatory mediators may be rel...