AN HMG-I PROTEIN FROM HUMAN ENDOTHELIAL-CELLS APPARENTLY IS SECRETED AND IMPAIRS ACTIVATION OF HAGEMAN-FACTOR (FACTOR-XII)

A highly purified protein from lysates of human umbilical vein endothe lial cells (HUVECs) inhibited the activation of factor XII [Hageman fa ctor (HF)] and removed factor XIIa from an activating surface, thus im pairing HF-dependent coagulation and kinin-releasing activities. Two t ryptic peptides from this protein had 100% identity with amino acids 3 1-44 and 89-101 of a nonhistone DNA-binding protein known as high-mobi lity group protein (HMG-I). In specific antibody experiments, the clot -inhibiting propel ty in purified lysate protein from HUVECs was assoc iated with HMG-I. The molecular weight of the protein that inhibited c lotting was consistent with that predicted for HMG-I. Protein that inh ibited contact activation and had antigenic properties of HMG-I and HU VEC lysate protein also was found in conditioned media from unchalleng ed cultured HUVECs. After HUVECs were incubated with C-14 lysine. cond itioned media contained immunoprecipitable radiolabeled protein with t he same molecular weight as that recovered from cell lysates, suggesti ng that this high-mobility group protein (HMG-I) may be secreted. Puri fied factor XII antigens were displaced from a glass surface by HMG-I from lysates in proportion to the amount of HMG-I protein that was add ed. This HMG-I probably inhibits factor XII functions because its high positive charge favors competitive binding to an activating substance .