Identification of haptoglobin as an angiogenic factor in sera from patients with systemic vasculitis.

Abstract

Angiogenesis is an important process in chronic inflammatory diseases.We observed that sera from patients with systemic vasculitis stimulated angiogenesis in an in vitro model using human umbilical vein endothelial cells cultured on a basement membrane (Matrigel) substrate.After 40% ammonium sulfate precipitation, angiogenic activity remained in the low molecular weight fraction and could be inactivated by heat.SDS-page of serum FPLC fractions exhibiting maximal angiogenic activity demonstrated two prominent species of 45 and 16-20 kD in patients' sera.These bands were much less apparent in sera obtained from control subjects.Amino-terminal sequencing of the 45-kD protein demonstrated that it was haptoglobin.Puri- fied haptoglobin stimulated angiogenesis in a dose-dependent manner.The angiogenic activity of vasculitis patients' sera was partially inhibited by an antihaptoglobin antibody.Further- more, serum haptoglobin levels in vasculitis patients correlated both with disease and angiogenic activity.Haptoglobin angio- genic activity was confirmed in two in vivo models using an implanted disc and a subcutaneous injection of basement mem- brane.Stimulation ofangiogenesis is a newly recognized biological function of haptoglobin.The increased levels of haptoglobin found in chronic inflammatory conditions may play an important role in tissue repair.In systemic vasculitis, haptoglobin might also compensate for ischemia by promoting development of collateral vessels.(J.Clin.Invest.1993.91:977-985.)Key words: vasculitis * inflammatory diseases * angiogenesis* base- ment membrane * endothelial cell ment membrane (2, 3).The basement membrane is an important biological mediator of this process and this has been ex- ploited in both in vitro and in vivo assays to assess the angio-