title: Tissue transglutaminase does not affect fibrotic matrix stability or regression of liver fibrosis in mice.
abstract: OBJECTIVE
  The ubiquitous cross-linking enzyme tissue transglutaminase (TG2) has been implicated in irreversible collagen stabilization in liver fibrosis, although functional evidence is lacking. We studied the contribution of TG2 to hepatic fibrotic matrix stability, as well as liver fibrosis progression and regression in TG2-deficient mice.
  
  
  METHODS
  Advanced liver fibrosis was induced by carbon tetrachloride or thioacetamide in TG2(-/-) mice and their wild-type littermates to study fibrosis progression and its spontaneous regression for up to 36 weeks. Pattern and extent of fibrosis were analyzed by histology and hepatic hydroxyproline quantification. Dynamic changes in hepatic matrix cross-linking were assessed by stepwise collagen extraction. Expression of 7 TGs and fibrosis-related genes was determined by quantitative reverse-transcription polymerase chain reaction.
  
  
  RESULTS
  Transglutaminase activity was increased in fibrosis, and the level of TG2 messenger RNA correlated with the expression of fibrosis-related genes. Biochemical analysis revealed progressive collagen stabilization, with an up to 6-fold increase in the highly cross-linked, pepsin-insoluble fraction (26%). In TG2(-/-) mice, hepatic TG activity was significantly decreased, but chronic administration of carbon tetrachloride or thioacetamide led to a comparable extent and pattern of liver fibrosis, as in wild-type mice. In TG2(-/-) mice, the composition of hepatic collagen fractions and levels of fibrosis-related transcripts were unchanged, and fibrosis reversal was not facilitated.
  
  
  CONCLUSIONS
  TG2 and TG activity are up-regulated during hepatic fibrosis progression, but do not contribute to fibrogenesis or stabilization of the collagen matrix. TG2 deletion does not promote regression of liver fibrosis. TG2-independent collagen cross-linking is a remarkable feature of progressing hepatic fibrosis and represents an important therapeutic target for liver fibrosis.
authors.affiliation: Division of Gastroenterology and Hepatology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA. ypopov@bidmc.harvard.edu
authors.forename: Yury
authors.initials: Y
authors.lastname: Popov
authors.forename: Deanna Y
authors.initials: DY
authors.lastname: Sverdlov
authors.forename: Anisha K
authors.initials: AK
authors.lastname: Sharma
authors.forename: K Ramakrishnan
authors.initials: KR
authors.lastname: Bhaskar
authors.forename: Shaoyong
authors.initials: S
authors.lastname: Li
authors.forename: Tobias L
authors.initials: TL
authors.lastname: Freitag
authors.forename: James
authors.initials: J
authors.lastname: Lee
authors.forename: Walburga
authors.initials: W
authors.lastname: Dieterich
authors.forename: Gerry
authors.initials: G
authors.lastname: Melino
authors.forename: Detlef
authors.initials: D
authors.lastname: Schuppan
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000235:genetics; D018450:Disease Progression; D005109:Extracellular Matrix / Q000378:metabolism / Q000473:pathology; D019204:GTP-Binding Proteins / Q000096:biosynthesis / Q000235:genetics*; D005786:Gene Expression Regulation; D008099:Liver / Q000201:enzymology / Q000473:pathology*; D008106:Liver Cirrhosis, Experimental / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D051379:Mice; D008810:Mice, Inbred C57BL; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D012313:RNA / Q000235:genetics*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D011503:Transglutaminases / Q000096:biosynthesis / Q000235:genetics*
chemical_list: D012313:RNA; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011503:Transglutaminases; D019204:GTP-Binding Proteins