Integrative cartilage repair: Inhibition by beta-aminopropionitrile

The effects of beta-aminopropionitrile, a known inhibitor of lysyl oxidase, on the extractability of newly synthesized collagen and integrative cartil age repair were determined in explant cultures of adult bovine articular ca rtilage. Dose-escalation studies indicated that treatment of cartilage expl ants for 6 days with beta-aminopropionitrile caused a dose-dependent inhibi tion of proteoglycan synthesis ([S-35]sulfate incorporation) with a 50% inh ibition at 2.2 mill. However, 0.25 mM beta-aminopropionitrile had no detect able effect on proteoglycan synthesis and was thus used for subsequent expe riments. Treatment of cartilage with beta-aminopropionitrile for 14 days in creased the extractability of newly synthesized collagen with 4 M guanidine -HCl while having little effect on proteoglycan synthesis, proteoglycan dep osition, collagen synthesis (formation of [H-3]hydroxyproline after labelin g with [H-3]proline), collagen deposition, or cartilage cellularity (DNA co ntent). In untreated cultures, the percentage of radiolabeled collagen ([H- 3]hydroxyproline) that was extractable after 1 day of radiolabeling, 6 days of radiolabeling, or 6 days of label and 6 days of chase decreased from 81 to 25 and 9%, respectively. In beta-aminopropionitrile-treated cultures, t he extractability was relatively higher (96, 62, and 47%, respectively). Tr eatment with beta-aminopropionitrile after radiolabeling with [C-14]lysine also significantly inhibited the formation of the reducible crosslink [C-14 ]dihydroxylysinonorleucine without affecting the overall deposition in cart ilage of [C-14]lysine and [C-14]hydroxylysine. In functional repair studies , treatment with beta-aminopropionitrile caused an almost complete inhibiti on of integration between pairs of cartilage explants maintained in apposit ion for 2 weeks. These results indicate that beta-aminopropionitrile blocks the formation of collagen crosslinks in cartilage explants and suggest tha t such crosslinks are critical to integrative cartilage repair.