Introduction The effect of low-intensity pulsed ultrasound (LIPUS) on cell growth was examined in three-dimensionalcultured chondrocytes with a collagen sponge. To elucidate the mechanisms underlying the mechanical activation of chondrocytes, intracellular signaling pathways through the Ras/ mitogen-activated protein kinase (MAPK) and the integrin/ phosphatidylinositol 3 kinase (PI3K)/Akt pathways as well as proteins involved in proliferation of chondrocytes were examined in LIPUS-treated chondrocytes.
Fibrosis is characterized by extracellular matrix (ECM) remodeling and stiffening. However, the functional contribution of tissue stiffening to noncancer pathogenesis remains largely unknown. Fibronectin (Fn) is an ECM glycoprotein substantially expressed during tissue repair. Here we show in advanced chronic liver fibrogenesis using a mouse model lacking Fn that, unexpectedly, Fn-null livers lead to more extensive liver cirrhosis, which is accompanied by increased liver matrix stiffness and deteriorated hepatic functions. Furthermore, Fnnull livers exhibit more myofibroblast phenotypes and accumulate highly disorganized/diffuse collagenous ECM networks composed of thinner and significantly increased number of collagen fibrils during advanced chronic liver damage. Mechanistically, mutant livers show elevated local TGF- activity and lysyl oxidase expressions. A significant amount of active lysyl oxidase is released in Fn-null hepatic stellate cells in response to TGF-1 through canonical and noncanonical Smad such as PI3 kinase-mediated pathways. TGF-1-induced collagen fibril stiffness in Fn-null hepatic stellate cells is significantly higher compared with wild-type cells. Inhibition of lysyl oxidase significantly reduces collagen fibril stiffness, and treatment of Fn recovers collagen fibril stiffness to wild-type levels. Thus, our findings indicate an indispensable role for Fn in chronic liver fibrosis/cirrhosis in negatively regulating TGF- bioavailability, which in turn modulates ECM remodeling and stiffening and consequently preserves adult organ functions. Furthermore, this regulatory mechanism by Fn could be translated for a potential therapeutic target in a broader variety of chronic fibrotic diseases.
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