Binding of human factors X and Xa to HepG2 and J82 human tumor cell lines. Evidence that factor Xa binds to tumor cells independent of factor Va.

Abstract

Previous studies demonstrated that several cultured human tumor cell lines potentiate the conversion of prothrombin to thrombin by factor Xa and calcium in the absence of exogenous factor Va. In the present study, the specific binding of radioiodinated preparations of human factor X and factor Xa to a human hepatocellular carcinoma cell line (HepG2) that constitutively synthesizes a factor V/Va molecule, and a human bladder carcinoma cell line (J82) that does not synthesize factor V/Va, was examined. Radioiodinated factor Xa bound specifically to J82 and HepG2 cells, whereas no significant specific binding of 125I-factor X to either cell was observed. The binding isotherm of 125I-factor Xa to each tumor cell line exhibited a hyperbolic profile, and Scatchard analysis demonstrated a single class of binding site for factor Xa on each cell surface with Kd values of 1.66 +/- 0.39 and 1.64 +/- 0.52 nM and 566,000 +/- 71,000 and 28,000 +/- 6,000 binding sites/cell for HepG2 and J82 cells, respectively. Thrombin formation by cell-bound factor Xa was hyperbolic and saturable at 5 nM factor Xa on each cell line. Hanes-Woolf plots of the prothrombin activation data indicated that half-maximal rates of thrombin formation occurred at factor Xa concentrations of 1.50 +/- 0.43 nM and 1.42 +/- 0.48 nM on HepG2 and J82 cells, respectively. Pretreatment of J82 cells with polyclonal anti-human factor V IgG had no measurable effect on either the binding of 125I-factor Xa or prothrombin activation. However, pretreatment of HepG2 cells with anti-human factor V IgG inhibited prothrombin activation in a dose-dependent manner, but did not inhibit the binding of factor Xa to this cell. When both cell lines were preincubated with exogenous human factor Va, the binding of factor Xa to either HepG2 or J82 cells was marginally affected. These data indicate that HepG2 and J82 cells have cell surface factor Xa binding sites proximal to, but independent of, cell surface factor Va of either exogenous or endogenous origin.