TGF‐β1 Suppresses Plasmin and MMP Activity in Flexor Tendon Cells via PAI‐1: Implications for Scarless Flexor Tendon Repair

Abstract

Flexor tendon injuries caused by deep lacerations to the hands are a challenging problem as they often result in debilitating adhesions that prevent the movement of the afflicted fingers. Evidence exists that tendon adhesions as well as scarring throughout the body are largely precipitated by the pleiotropic growth factor, TGF-β1, but the effects of TGF-β1 are poorly understood in tendon healing. Using an in vitro model of tendon healing, we previously found that TGF-β1 causes gene expression changes in tenocytes that are consistent with scar tissue and adhesion formation, including upregulation of the anti-fibrinolytic protein, PAI-1. Therefore, we hypothesized that TGF-β1 contributes to scarring and adhesions by reducing the activity of proteases responsible for ECM degradation and remodeling, such as plasmin and MMPs, via upregulation of PAI-1. To test our hypothesis, we examined the effects of TGF-β1 on the protease activity of tendon cells. We found that flexor tendon tenocytes treated with TGF-β1 had significantly reduced levels of active MMP-2 and plasmin. Interestingly, the effects of TGF-β1 on protease activity were completely abolished in tendon cells from homozygous PAI-1 KO mice, which are unable to express PAI-1. Our findings support the hypothesis that TGF-β1 induces PAI-1, which suppresses plasmin and plasmin-mediated MMP activity, and provide evidence that PAI-1 may be a novel therapeutic target for preventing adhesions and promoting a scarless, regenerative repair of flexor tendon injuries.

Tendon development and musculoskeletal assembly: emerging roles for the extracellular matrix

“…Collagenases such as Mmp1 target the structural collagens for degradation, whereas gelatinases such as Mmp2 and Mmp9 and membrane-bound MMPs, such as Mmp14, degrade smaller network collagens. Importantly, recent studies have shown that the expression and activity of MMPs are regulated by signals activated in response to mechanical forces, such as Itg and Tgfβ (Yu and Stamenkovic, 2000;Farhat et al, 2015). MMP misregulation also occurs upon tendon inflammation, and recent studies suggest that MMP inhibition can improve tendon repair (Bedi et al, 2010;Jelinsky et al, 2011;Farhat et al, 2012;Davis et al, 2013).…”

“…Injuries lead to excessive release of TGFβ owing to mechanical forcemediated activation of TGFβ (discussed below) and can cause fibrotic scarring of the tendon, thereby disrupting its function (Farhat et al, 2015). In one model, elevated TGFβ might overactivate MMPs, which in turn promotes further release of active TGFβ from the ECM as well as activating expression of Scx and Mkx and driving further ECM production.…”

“…5) (Liu et al, 2015). Notably, TGFβ signaling in response to mechanical forces also controls MMP expression during tendon repair (Katzel et al, 2011;Farhat et al, 2015). Interestingly, injured tendons in Egr1 −/− mutant mice fail to upregulate TGFβ and Scx or to repair their tendons efficiently (Guerquin et al, 2013).…”

“…Similarly, in vitro studies of mechanical loading on mouse tenocytes have shown that, whereas low levels of shear force lead to upregulation of Col1a and Tmnd, increasing the force leads to upregulation of Runx2 and Sox9 (Zhang and Wang, 2015). Understanding these dynamic responses to mechanical stimulation might lead to improved therapeutic interventions; the systemic inhibition of MMPs, for example, can reduce fibrotic scarring of muscle ECM (Farhat et al, 2015). MMP expression is also regulated by TGFβs (Yu and Stamenkovic, 2000;Ge and Greenspan, 2006;Farhat et al, 2015).…”

“…Understanding these dynamic responses to mechanical stimulation might lead to improved therapeutic interventions; the systemic inhibition of MMPs, for example, can reduce fibrotic scarring of muscle ECM (Farhat et al, 2015). MMP expression is also regulated by TGFβs (Yu and Stamenkovic, 2000;Ge and Greenspan, 2006;Farhat et al, 2015). In turn, MMP2, MMP9 and BMP1 proteases might specifically digest LLC and release active TGFβ, establishing a positive-feedback loop that could help finetune MMP levels, both during normal tendon function and in response to tendon injury or exercise (Fig.…”