Transglutaminase-mediated cross-linking is involved in the stabilization of extracellular matrix in human liver fibrosis

Background/Aims: Lysyl oxidase-mediated cross-linking contributes to the st abilization of collagen in liver fibrosis. We have investigated transglutam inase-mediated cross-linking, to determine if it participates in the stabil ization of extracellular matrix in human liver fibrosis. Methods: Transglutaminase activity was assessed in vitro by incorporation o f biotinylated amine into liver proteins. The product of the transglutamina se-catalyzed cross-linking reaction, N-epsilon(gamma -glutamyl)lysine, and the extracellular proteins cross-linked by it, were localized by immunohist ochemistry in fibrotic livers. The cross-linked complexes were extracted fr om liver tissue, immunopurified and characterized by Western blot. Results: Transglutaminase, detected by immunohistochemistry, Western blot a nd by enzymatic activity, was found in higher amounts in fibrotic than in n ormal liver. The N-epsilon(gamma -glutamyl)lysine cross-link, undetectable in normal liver, was present extracellularly in fibrotic liver, where it wa s co-distributed with osteonectin, mostly in inflammatory areas submitted t o an intense remodeling. Cross-linking of osteonectin by transglutaminase w as confirmed by Western blot. In parasitic fibrosis transglutaminase also o riginates from the parasite. Conclusions: Transglutaminase-mediated cross-linking occurs in liver extrac ellular matrix during the early, inflammatory, stage of liver fibrosis, whe reas cross-linking by pyridinoline occurs mostly later in the fibrotic proc ess. This could lead to the development of new anti-fibrotic treatments tar geted to a specific stage of fibrosis. (C) 2001 European Association for th e Study of the Liver. Published by Elsevier Science B.V. All rights reserve d.