Chronic liver injury drives non‐traditional intrahepatic fibrin(ogen) crosslinking via tissue transglutaminase

Abstract

Introduction: Intravascular fibrin clots and extravascular fibrin deposits are often implicated in the progression of liver fibrosis. However, evidence supporting a pathological role of fibrin in hepatic fibrosis is indirect and based largely on studies using anticoagulant drugs that inhibit activation of the coagulation protease thrombin, which has other downstream targets that promote fibrosis. Therefore, the goal of this study was to determine the precise role of fibrin deposits in experimental hepatic fibrosis. Methods: Liver fibrosis was induced in mice expressing mutant fibrinogen insensitive to thrombin-mediated proteolysis (i.e., locked in the monomeric form), termed FibAEK mice, and Factor XIII A2 subunit-deficient mice (FXIII−/−). Female wild-type mice, FXIII−/− mice, and homozygous FibAEK mice were challenged with carbon tetrachloride (CCl4), twice weekly for 4 or 6 weeks (1 ml/kg, ip). Results: Hepatic injury and fibrosis induced by CCl4 challenge were unaffected by FXIII deficiency or inhibition of thrombin-catalyzed fibrin polymer formation (in FibAEK mice). Surprisingly, hepatic deposition of cross-linked fibrin(ogen) was not reduced in CCl4-challenged FXIII−/− mice or FibAEK mice compared to wild-type mice. Rather, deposition of cross-linked hepatic fibrin(ogen) following CCl4 challenge was dramatically reduced in tissue transglutaminase-deficient mice (TGM2−/− mice). However, the reduction in cross-linked fibrin(ogen) in TGM2−/− mice did not affect CCl4-induced liver fibrosis. Conclusions: These results indicate that neither traditional fibrin clots, formed by the thrombin:FXIIIa pathway, nor atypical TGM2-cross-linked fibrin(ogen) contribute to experimental CCl4-induced liver fibrosis. Collectively, the results indicate that liver fibrosis occurs independently of intrahepatic fibrin(ogen) deposition.

Liver fibrosis is driven by protease‐activated receptor‐1 expressed by hepatic stellate cells in experimental chronic liver injury

“…53 Future studies should investigate differential activation of hepatic stellate cells by these 2 different PAR-1 agonists. Finally, there is also a need to better evaluate intrahepatic thrombin activity in the injured liver.Whereas numerous studies use intrahepatic fibrin(ogen) deposition as an index of hepatic thrombin activity, our recent studies indicate this may not be a reliable indicator of intrahepatic coagulation 14.…”

“…Hepatic fibrosis was quantified using multiple complementary approaches as described previously. 14 For assessment of chronic liver injury, paraffin-embedded liver sections were stained with hematoxylin and eosin and examined by a board-certified veterinary pathologist (KJW) in a blinded fashion.…”

“…13 Although the role of fibrin(ogen) in liver fibrosis is likely context dependent, prior studies indicate that thrombin-catalyzed fibrin polymerization does not contribute to hepatic fibrosis induced by chronic carbon tetrachloride (CCl 4 ) challenge. 14 A second mechanism whereby coagulation protease activity may drive hepatic fibrosis is through activation of G-protein-coupled protease-activated receptors (PARs). Whole-body deficiency or pharmacologic inhibition of either PAR-1 or PAR-2 has been shown to reduce experimental hepatic fibrosis in several studies.…”

Aβ peptide and fibrinogen weave a web of destruction in cerebral amyloid angiopathy

“…Indeed, both molecules may be present as monomers or fibrillar polymers and the relative amount of each species could be quite heterogenous. Recent analyses on the contribution of fibrin(ogen) to liver injury have suggested that not only are deposits of fibrinogen, fibrin, and cross-linked fibrin present within injured zones but that each molecule exerts unique effects on cells in the microenvironment to influence distinct aspects of tissue injury, remodeling, and repair (18). It is possible that similar unique fibrin(ogen)-driven contributions could be made in AD.…”

Crosslinked clots formed independently of factor XIII and without fibrinogen‐to‐fibrin conversion – is this a liver‐specific phenomenon?

“…In this issue of the Journal of Thrombosis and Haemostasis , Poole et al . present data that made me wonder whether everything I have been taught in the 20 years in which I have studied thrombosis and hemostasis is wrong . In a mouse model of chronic liver injury, it was demonstrated that crosslinked fibrin(ogen) is deposited in a process that does not require fibrinogen‐to‐fibrin conversion or factor XIII.…”