title: A novel mechanism of UV-A and riboflavin-mediated corneal cross-linking through induction of tissue transglutaminases.
abstract: OBJECTIVE
  Collagen cross-linking using UV-A irradiation combined with the photosensitizer riboflavin is a new technique for treating progressive keratoconus. The purposes of this study were to examine whether primary human corneal keratocytes (HCKs) are capable of expressing and secreting fibronectin and tissue transglutaminase (tTgase), an enzyme cross-linking extracellular matrix protein, and to examine whether fibronectin and tTgase are increased after the treatment of HCK cells with UV-A irradiation combined with riboflavin (RFUV-A), thus providing another possible physiological mechanism of the cross-linking pathway.
  
  
  METHODS
  Cell cultures established from HCKs were treated with 0.025% riboflavin solution and UV-A (370 nm) irradiance 3 mW/cm2 for 30 minutes. Induction of fibronectin and tTgase was investigated by immunohistochemistry, real-time polymerase chain reaction, and Western blot analysis. Cell viability was quantified by a microscopic live-dead assay. External tTgase activity was measured by the ability to form polymerized fibronectin and the incorporation of biotinylated cadaverine into fibronectin.
  
  
  RESULTS
  Treatment of cultured HCK cells with RFUV-A increased the fibronectin and tTgase messenger RNA and protein levels. This effect was not observed in cells treated with riboflavin or UV-A radiation alone. Incorporation of biotinylated cadaverine was significantly increased when HCK cells were treated with RFUV-A.
  
  
  CONCLUSIONS
  The enzymes tTgase and fibronectin are expressed by RFUV-A treatment in cultured HCK cells. This mechanism provides more information about the physiology of corneal cross-linking.
authors.affiliation: Moorfields Eye Hospital, London, United Kingdom. Nick.Kopsachilis@moorfields.nhs.uk
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Kopsachilis
authors.forename: Konstantinos T
authors.initials: KT
authors.lastname: Tsaousis
authors.forename: Ioannis T
authors.initials: IT
authors.lastname: Tsinopoulos
authors.forename: Friedrich E
authors.initials: FE
authors.lastname: Kruse
authors.forename: Ulrich
authors.initials: U
authors.lastname: Welge-Luessen
mesh_terms: D015153:Blotting, Western; D002470:Cell Survival; D002478:Cells, Cultured; D003094:Collagen / Q000378:metabolism*; D060527:Corneal Keratocytes / Q000187:drug effects / Q000201:enzymology* / Q000528:radiation effects; D003319:Corneal Stroma / Q000166:cytology; D003432:Cross-Linking Reagents / Q000494:pharmacology*; D005353:Fibronectins / Q000096:biosynthesis / Q000235:genetics; D019204:GTP-Binding Proteins / Q000096:biosynthesis* / Q000235:genetics; D006801:Humans; D017319:Photosensitizing Agents / Q000494:pharmacology*; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D012333:RNA, Messenger / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D012256:Riboflavin / Q000494:pharmacology*; D011503:Transglutaminases / Q000096:biosynthesis* / Q000235:genetics; D014466:Ultraviolet Rays
chemical_list: D003432:Cross-Linking Reagents; D005353:Fibronectins; D017319:Photosensitizing Agents; D012333:RNA, Messenger; D003094:Collagen; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D011503:Transglutaminases; D019204:GTP-Binding Proteins; D012256:Riboflavin