Fibronectin Protects from Excessive Liver Fibrosis by Modulating the Availability of and Responsiveness of Stellate Cells to Active TGF-β

Abstract

Fibrotic tissue in the liver is mainly composed of collagen. Fibronectin, which is also present in fibrotic matrices, is required for collagen matrix assembly in vitro. It also modulates the amount of growth factors and their release from the matrix. We therefore examined the effects of the absence of fibronectin on the development of fibrosis in mice.Conditional deletion of fibronectin in the liver using the Mx promoter to drive cre expression resulted in increased collagen production and hence a more pronounced fibrosis in response to dimethylnitrosamine in mice. Exclusive deletion of fibronectin in hepatocytes or normalization of circulating fibronectin in Mx-cKO mice did not affect the development of fibrosis suggesting a role for fibronectin production by other liver cell types. The boosted fibrosis in fibronectin-deficient mice was associated with enhanced stellate cell activation and proliferation, elevated concentrations of active TGF-β, and increased TGF-β-mediated signaling.In vitro experiments revealed that collagen-type-I production by fibronectin-deficient hepatic stellate cells stimulated with TGF-β was more pronounced, and was associated with augmented Smad3-mediated signaling. Interfering with TGF-β signaling using SB431542 normalized collagen-type-I production in fibronectin-deficient hepatic stellate cells. Furthermore, precoating culture plates with fibronectin, but not collagen, or providing fibronectin fibrils unable to interact with RGD binding integrins via the RGD domain significantly diminished the amount of active TGF-β in fibronectin-deficient stellate cells and normalized collagen-type-I production in response to TGF-β stimulation. Thus, excessive stellate cell activation and production of collagen results from increased active TGF-β and TGF-β signaling in the absence of fibronectin.In conclusion, our data indicate that fibronectin controls the availability of active TGF-β in the injured liver, which impacts the severity of the resulting fibrosis. We therefore propose a novel role for locally produced fibronectin in protecting the liver from an excessive TGF-β-mediated response.

Inhibition of fibronectin deposition improves experimental liver fibrosis

“…In vitro experiments in HSCs were performed on cells isolated from wild type mice or transgenic mice whose HSCs either do not produce fibronectin (FN À/À ) or produce a mutated fibronectin, able to form short fibrils but unable to activate RGD-binding integrins (because of the presence of an RGD-to-RGE mutation) (FN À/RGE ) [15,19,20]. Wild type C57BL/6 mice (Charles-River) were used for injection of pUR4 and CT-peptides.…”

“…In conditional knockout mice, unable to produce fibronectin in the liver, the decrease in fibronectin production is associated with an increase in available active-TGF-b, and hence an increase in the activation of HSCs, collagen production and matrix accumulation [15]. However, lack of fibronectin and inability of fibronectin to assemble into fibrils may have distinct effects on fibrosis.…”

Endocytosis of collagen by hepatic stellate cells regulates extracellular matrix dynamics

“…Upon liver injury, they are transformed to active myofibroblast-like cells, which are characterized by increased proliferation, ␣-smooth muscle actin expression, and robust collagen production (3-5, 13, 19, 26, 31). However, matrix also reciprocally regulates HSC function, including adhesion and migration through mechanical stiffness, integrin activation, and other pathways (2,15,20,39,57). However, the mechanisms by which HSCs sense their matrix surroundings are incompletely understood.…”

Molecular Mechanism Responsible for Fibronectin-controlled Alterations in Matrix Stiffness in Advanced Chronic Liver Fibrogenesis

“…into the ECM (16,40), and local TGF-␤ activity is elevated in Fn-null livers in initial stages following liver injury (26,41). Hence, the effect of Fn deficiency on the production of latent complexes of TGF-␤ and local TGF-␤ bioavailability during advanced chronic liver injury was also investigated.…”