title: A novel binding site for the native hepatic acute-phase protein alpha1-antitrypsin expressed on the human hepatoma cell line HepG 2 and intestinal cell line Caco 2.
abstract: OBJECTIVE
  alpha1-antitrypsin (alpha1-AT) is a hepatic acute phase protein which predominantly inhibits neutrophil elastase. Besides this major function, we have also previously shown that alpha1-AT markedly increased H-ferritin mRNA expression and ferritin synthesis in the human hepatoma cell line HepG 2. These actions suggest that alpha1-AT might interact with HepG 2 cells via a specific cell surface binding site.
  
  
  RESULTS
  Using radio-labelled native alpha1-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/10(6) cells equivalent to 195,800+/-29,200 sites/cell. The binding of [125I]alpha1-AT was time dependent with a calculated association rate constant of 9.22+/-1.84x10(4)xM(-1)xmin(-1). Binding was highly specific since other acute phase proteins or protease inhibitors failed to block binding. Although alpha1-AT-trypsin, alpha1-AT-elastase and the pentapeptide FVYLI, the minimal binding sequence for the SEC receptor, increased [125I]alpha1-AT binding, in long term experiments these complexes failed to influence the number of alpha1-AT binding sites. Specific, saturable binding of [125I]alpha1-AT was also found on the human intestinal epithelial Caco 2 cells, but not on fibroblast or leukaemic cell lines.
  
  
  CONCLUSIONS
  These experiments demonstrate a specific, high affinity binding site for native alpha1-AT on HepG 2 and Caco 2 cells, cell lines derived from tissues involved in the acute phase response.
authors.affiliation: Institute of Liver Studies, King's College School of Medicine and Dentistry, London, UK.
authors.forename: I
authors.initials: I
authors.lastname: Graziadei
authors.forename: W
authors.initials: W
authors.lastname: Vogel
authors.forename: A
authors.initials: A
authors.lastname: Bomford
mesh_terms: D001665:Binding Sites; D018938:Caco-2 Cells / Q000378:metabolism*; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D006801:Humans; D014407:Tumor Cells, Cultured; D000515:alpha 1-Antitrypsin / Q000378:metabolism*
chemical_list: D000515:alpha 1-Antitrypsin