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Keywords:

  • α1-antitrypsin;
  • binding site;
  • HepG2 cells;
  • Caco 2 cells;
  • serpin-enzyme complex;
  • receptor;
  • ferritin

Abstract:Aims/Background:α1-antitrypsin (α1-AT) is a hepatic acute phase protein which predominantly inhibits neutrophil elastase. Besides this major function, we have also previously shown that α1-AT markedly increased H-ferritin mRNA expression and ferritin synthesis in the human hepatoma cell line HepG 2. These actions suggest that α1-AT might interact with HepG 2 cells via a specific cell surface binding site. Methods and Results: Using radio-labelled native α1-AT, we observed saturable binding to HepG 2 cells with a dissociation constant (Kd) of 63.3±6.9 nM and a maximal density of binding sites (Bmax) of 0.34 ±0.05 pmol/106 cells equivalent to 195800±29200 sites/cell. The binding of [125I]α1-AT was time dependent with a calculated association rate constant of 9.22±1.84×104×M−1×min−1. Binding was highly specific since other acute phase proteins or protease inhibitors failed to block binding. Although α1-AT-trypsin, α1-AT-elastase and the pentapeptide FVYLI, the minimal binding sequence for the SEC receptor, increased [125I]α1-AT binding, in long term experiments these complexes failed to influence the number of α1-AT binding sites. Specific, saturable binding of [125I]α1-AT was also found on the human intestinal epithelial Caco 2 cells, but not on fibroblast or leukaemic cell lines. Conclusion: These experiments demonstrate a specific, high affinity binding site for native α1-AT on HepG 2 and Caco 2 cells, cell lines derived from tissues involved in the acute phase response.