A novel binding site for the native hepatic acute-phase protein alpha 1-antitrypsin expressed on the human hepatoma cell line HepG 2 and intestinal cell line Caco 2

Aims/Background: alpha 1-antitrypsin (alpha 1-AT) is a hepatic acute phase protein which predominantly inhibits neutrophil elastase. Besides this majo r function, Eve have also previously shown that oil-AT markedly increased H -ferritin mRNA expression and ferritin synthesis in the human hepatoma cell line HepG 3. These actions suggest that alpha 1-AT might interact with Hep G 2 cells via a specific cell surface binding site, Methods and Results: Us ing radio-labelled native alpha 1-AT, we observed saturable binding to HepG 2 cells with a dissociation constant (Kd) of 63.3+/-6.9 nhl and a maximal density of binding sites (Bmax) of 0.34 +/-0.05 pmol/10(6) cells equivalent to 195800+/-29200 sites/cell. The binding of [I-125]alpha 1-AT was time de pendent 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 o r protease inhibitors failed to block binding. Although alpha 1-AT-trypsin, alpha 1-AT-elastase and the pentapeptide FVYLI, the minimal binding sequen ce for the SEC receptor, increased [I-125]alpha 1-AT binding, in long term experiments these complexes failed to influence the number of alpha 1-AT bi nding sites. Specific, saturable binding of [I-125]alpha 1-AT was also foun d 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 alpha 1-AT on HepG 2 and Caco 2 cel ls, cell lines derived from tissues involved in the acute phase response.