title: The role of nonenzymatic glycation and carbonyls in collagen cross-linking for the treatment of keratoconus.
abstract: OBJECTIVE
  Corneal cross-linking (CXL) is a treatment for keratoconus that eliminates the need for keratoplasty in most patients. However, its molecular mechanisms remain under study. Advanced glycation end products (AGEs) have been suggested by many studies as the causative strengthening agent during CXL, though no studies to date have directly tested this hypothesis.
  
  
  METHODS
  Corneas of young rabbits and sharks were pretreated with pyridoxal hydrochloride and copper ions before CXL. Two known inhibitors of AGE formation, aminoguanidine and rifampicin, were applied during CXL in the treatment solution. Tensile strength tests were conducted after these experiments to detect diminished or accentuated corneal stiffening after CXL. SDS-PAGE was performed on type I collagen cross-linked in the absence and presence of AGE inhibitors.
  
  
  RESULTS
  Pretreatment with pyridoxal hydrochloride resulted in significantly higher corneal stiffening after CXL. AGE inhibitors significantly diminished cross-linking as detected by both tensile strength measurements using whole corneas and gel electrophoresis of in vitro cross-linking of type I collagen in solution, in the presence and absence of the inhibitors. Rifampicin inhibited CXL more significantly than aminoguanidine in gel electrophoresis and tensile strength tests, confirming recent findings on its efficacy as an AGE inhibitor.
  
  
  CONCLUSIONS
  Data presented here suggest that CXL is carbonyl dependent and involves the formation of AGE cross-links. Six possible cross-linking mechanisms are discussed.
authors: Division of Biology, Kansas State University, Manhattan, Kansas 66506-4901, USA. gadbrum@ksu.edu
authors: Gage
authors: G
authors: Brummer
authors: Stacy
authors: S
authors: Littlechild
authors: Scott
authors: S
authors: McCall
authors: Yuntao
authors: Y
authors: Zhang
authors: Gary W
authors: GW
authors: Conrad
mesh_terms: D000818:Animals; D024042:Collagen Type I / Q000378:metabolism*; D003300:Copper / Q000494:pharmacology; D003319:Corneal Stroma / Q000378:metabolism; D003432:Cross-Linking Reagents; D004284:Dogfish; D004591:Electrophoresis, Polyacrylamide Gel; D017127:Glycation End Products, Advanced / Q000037:antagonists & inhibitors / Q000502:physiology*; D006146:Guanidines / Q000494:pharmacology; D007640:Keratoconus / Q000188:drug therapy* / Q000378:metabolism; D017319:Photosensitizing Agents / Q000627:therapeutic use*; D050050:Protein Carbonylation / Q000502:physiology*; D011730:Pyridoxal / Q000494:pharmacology; D011817:Rabbits; D012256:Riboflavin / Q000627:therapeutic use*; D012293:Rifampin / Q000494:pharmacology; D013718:Tensile Strength; D014466:Ultraviolet Rays
chemical_list: D024042:Collagen Type I; D003432:Cross-Linking Reagents; D017127:Glycation End Products, Advanced; D006146:Guanidines; D017319:Photosensitizing Agents; D011730:Pyridoxal; D003300:Copper; C004479:pimagedine; D012256:Riboflavin; D012293:Rifampin