PAI‐1 mediates TGF‐β1‐induced myofibroblast activation in tenocytes via mTOR signaling

Abstract

Transforming growth factor‐beta (TGF‐β1) induces plasminogen activator inhibitor 1 (PAI‐1) to effect fibrotic pathologies in several organs including tendon. Recent data implicated PAI‐1 with inhibition of phosphatase and tensin homolog (PTEN) suggesting that PAI‐1‐induced adhesions involves phosphoinositide 3‐kinase/protein kinase B/mammalian target of rapamycin (mTOR) signaling. Ergo, we investigated effects of TGF‐β1, PAI‐1, and mTOR signaling crosstalk on myofibroblast activation, senescence, and proliferation in primary flexor tenocytes from wild‐type (WT) and PAI‐1 knockout (KO) mice. PAI‐1 deletion blunted TGF‐β1‐induced myofibroblast activation in murine flexor tenocytes and increased the gene expression of Mmp‐2 to confer protective effects against fibrosis. While TGF‐β1 significantly reduced phosphorylation of PTEN in WT cells, PAI‐1 deletion rescued the activation of PTEN. Despite that, there were no differences in TGF‐β1‐induced activation of mTOR signaling (AKT, 4EBP1, and P70S6K) in WT or KO tenocytes. Phenotypic changes in distinct populations of WT or KO tenocytes exhibiting high or low mTOR activity were then examined. TGF‐β1 increased alpha‐smooth muscle actin abundance in WT cells exhibiting high mTOR activity, but this increase was blunted in KO cells exhibiting high 4EBP1 activity but not in cells exhibiting high S6 activity. DNA damage (γH2AX) was increased with TGF‐β1 treatment in WT tenocytes but was blunted in KO cells exhibiting high mTOR activity. Increased mTOR activity enhanced proliferation (Ki67) in both WT and KO tenocytes. These findings point to a complex nexus of TGF‐β1, PAI‐1, and mTOR signaling in regulating proliferation, myofibroblast differentiation, and senescence in tenocytes, which could define therapeutic targets for chronic tendon adhesions and other fibrotic pathologies.

Emergent Peptides of the Antifibrotic Arsenal: Taking Aim at Myofibroblast Promoting Pathways

“…Furthermore, in ex vivo lung tissue cultures, E4 induced MMP-1 activity and reduced collagen I, fibronectin, and PAI-1. Indeed, previous studies also indicated the roles of uPAR in attenuating myofibroblast functions, and these included proliferation, recruitment, and the subsequent progression of renal fibrosis by regulating extracellular signal-regulated kinase (ERK) signaling and reducing the extracellular accumulation of PAI-1 [ 36 ], which are also well-documented downstream targets of TGF-β1 signaling [ 11 , 37 , 38 ].…”

SERPINE1: Role in Cholangiocarcinoma Progression and a Therapeutic Target in the Desmoplastic Microenvironment

“…The available data further suggest, moreover, that PAI-1 is a key factor in the TGF-β1driven transformation of fibroblasts to a myofibroblastic phenotype. Indeed, flexor tenocyte myofibroblast differentiation in response to TGF-β1 is significantly attenuated in SERPINE1deficient mice [112], and incubation with SK-216, a pharmacologic inhibitor of PAI-1 activity, effectively blunts the TGF-β1-induced transition of MRC-5 fibroblasts to myofibroblasts as well as the epithelial-to-mesenchymal conversion in TGF-β1-treated A549 lung epithelial cells [113]. PAI-1 binding to urokinase also induces corneal myofibroblast differentiation on a vitronectin substrate [114], while the treatment of dorsal skin wounds in FVB/NJ mice with the small molecule PAI-1 antagonist Tiplaxtinin dramatically reduced wound closure and re-epithelialization [115].…”

The roles and mechanisms of the NF-κB signaling pathway in tendon disorders

“…They initially appear in the proliferative phase to synthesize and contract the ECM to close gaps within injured tendons ( 76 ). However, the persistence of myofibroblasts is recognized as a putative driver of fibrovascular scar healing in the tendon tissue due to exuberant and sustained ECM production ( 77 ). In the flexor tendon healing, they also lead to tendon adhesions between the injured site and the surrounding tissue, preventing tendon gliding ( 78 ).…”